ci: switch Go to stable/oldstable (& coveralls adjustment)

Author: philpennock

.github/workflows/pushes.yaml

@@ -20,9 +20,9 @@ jobs:
     strategy:
       matrix:
         include:
-          - go: '1.21.x'
+          - go: 'stable'
             canonical: true
-          - go: '1.20.x'
+          - go: 'oldstable'
             canonical: false
 
     steps:
@@ -106,7 +106,7 @@ jobs:
         uses: shogo82148/actions-goveralls@v1
         with:
           path-to-profile: ${{ runner.temp }}/profile.cov
-          flag-name: Go-${{ matrix.go }}
+          flag-name: ${{ steps.go-settings.outputs.go-version }}
           parallel: true