Run build on macos-13

macos-11 no longer seems to be supported, see:

    https://docs.github.com/en/actions/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories

Build indefinitely stays in "Waiting for a runner to pick up this job..."

.github/workflows/build.yml

@@ -162,7 +162,7 @@ jobs:
 
   build_macos:
     name: "Build: MacOS"
-    runs-on: macos-11
+    runs-on: macos-13
     strategy:
       matrix:
         arch:
@@ -190,7 +190,7 @@ jobs:
 
       # Select matching Xcode and SDK, see: https://xcodereleases.com/
       - run: |
-          sudo xcode-select -switch /Applications/Xcode_12.5.1.app
+          sudo xcode-select -switch /Applications/Xcode_15.2.app
           echo "SDKROOT=$(xcrun --show-sdk-path)" >> $GITHUB_ENV
 
       - name: Setup gfortran