Use the Snap versioning automation of ubuntu/desktop-snaps (#14)

Remove the versioning automation scripting from snapcraft.yaml and add
the needed changes so that the GitHub action of ubuntu/desktop-snaps
does the versioning automation.

- Hard-code "version" and "grade" metadata fields in
  snapcraft.yaml. The GitHub action modifies this file to bump the
  version. This also improves readability, having the current version
  number visible near the top of the file.

- Remove "override-pull:" in the "ghostscript" part. We use the
  versioning automation in the GitHub action and do not need our own
  script any more.

- Remove the "jq" from the "build-packages:" in the "ghostscript"
  part. This was only needed for our own versioning automation script.

- In the workflow file, .github/workflows/auto-update.yml, we add the
  parameter "version-schema: '^ghostpdl-(\d+\.\d+\.\d+)'" to the call
  of the GitHub action. This tells to the action to also perform
  versioning automation and it also tells the format of the upstream
  release tags.

- We do not remove the "adopt-info: ghostscript" even with all
  metadata fields in snapcraft.yaml being hard-coded and no "craftctl
  set ..." calls in the "ghostscript" part. It only tells the GitHub
  action which part is the supplier of the upstream version number.

.github/workflows/auto-update.yml

@@ -17,4 +17,4 @@ jobs:
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
           repo: ${{ github.repository }}
-
+          version-schema: '^ghostpdl-(\d+\.\d+\.\d+)'

snap/snapcraft.yaml

@@ -1,5 +1,7 @@
 name: ghostscript-printer-app
 base: core22
+version: '10.02.1-5'
+grade: stable
 summary: Ghostscript Printer Application
 description: |
   The Ghostscript Printer Application is a PAPPL (Printer Application
@@ -214,70 +216,7 @@ parts:
       - --enable-freetype
       - --without-tesseract
       - --datarootdir=/snap/ghostscript-printer-app/current/usr/share/
-    override-pull: |
-      set -eux
-      # Do the actual pull task
-      craftctl default
-      # Settings:
-      # Force channel: auto/devel/edge/stable
-      CHANNEL=stable
-      # Force package release number (integer) or "auto"
-      PACKAGERELEASE=auto
-      # Time stamp of Snap build in Snap Store
-      snapbuilddatehuman=`curl -s -H 'Snap-Device-Series: 16' https://api.snapcraft.io/v2/snaps/info/ghostscript-printer-app | jq -r '."channel-map" | .[] | select(.channel.name == "edge") | select(.channel.architecture == "amd64") | ."created-at"'`
-      snapbuilddate=`date +%s --date=$snapbuilddatehuman`
-      if [ -z "$snapbuilddate" ]; then
-          snapbuilddate=0
-      fi
-      # Time stamp of the last GIT commit of the snapping repository
-      pushd $CRAFT_PROJECT_DIR
-      gitcommitdate=`git log -1 --date=unix | grep Date: | perl -p -e 's/Date:\s*//'`
-      popd
-      if [ -z "$gitcommitdate" ]; then
-          gitcommitdate=0
-      fi
-      # Previous stable and development version
-      prevstable="$(curl -s -H 'Snap-Device-Series: 16' https://api.snapcraft.io/v2/snaps/info/ghostscript-printer-app | jq -r '."channel-map" | .[] | select(.channel.name == "stable") | select(.channel.architecture == "'$CRAFT_TARGET_ARCH'") | .version')"
-      if [ -z "$prevstable" ]; then
-          prevstable=0
-      fi
-      prevdevel="$(curl -s -H 'Snap-Device-Series: 16' https://api.snapcraft.io/v2/snaps/info/ghostscript-printer-app | jq -r '."channel-map" | .[] | select(.channel.name == "edge") | select(.channel.architecture == "'$CRAFT_TARGET_ARCH'") | .version')"
-      if [ -z "$prevdevel" ]; then
-          prevdevel=0
-      fi
-      # Previous version in general
-      dpkg --compare-versions "$prevdevel" lt "$prevstable" && prevversion=$prevstable || prevversion=$prevdevel
-      # Current upstream version of ghostscript
-      upstreamversion="$(git describe --tags --always | sed -E 's/ghostpdl-([0-9]+\.[0-9]+\.[0-9]+).*/\1/')"
-      # Determine package release number
-      if test "x$PACKAGERELEASE" = "xauto"; then
-          packagerelease=`echo "$prevversion" | perl -p -e 's/^('"$upstreamversion"'\-(\d+)|.*)$/\2/'`
-          if [ -z "$packagerelease" ]; then
-              packagerelease=1
-          else
-              if test "$gitcommitdate" -gt "$snapbuilddate"; then
-                  packagerelease=$(( $packagerelease + 1 ))
-              fi
-          fi
-      else
-          packagerelease=$PACKAGERELEASE
-      fi
-      # Compose version string
-      version="$upstreamversion-$packagerelease"
-      # Select channel
-      if test "x$CHANNEL" = "xedge" -o "x$CHANNEL" = "xdevel"; then
-          grade=devel
-      elif test "x$CHANNEL" = "xstable"; then
-          grade=stable
-      else
-          [ -n "$(echo $version | grep "+git")" ] && grade=devel || grade=stable
-      fi
-      # Set version and grade
-      craftctl set version="$version"
-      craftctl set grade="$grade"
-      export SNAPCRAFT_PROJECT_VERSION="$version"
     build-packages:
-      - jq
       - curl
     stage-packages:
       - libpaper1