gha: test against go1.13 (oldest), go1.21 and go1.22

Keeping the oldest version in the matrix, as that's defined
as minimum in go.mod, but testing against latest and previous.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

.github/workflows/ci.yaml

@@ -13,7 +13,7 @@ jobs:
     timeout-minutes: 10
     strategy:
       matrix:
-        go-version: [1.13.x, 1.18.x, 1.19.x]
+        go-version: [1.13.x, 1.21.x, 1.22.x]
         platform: [ubuntu-latest]
     runs-on: ${{ matrix.platform }}
     steps: