gha: update to test against Go "oldest", stable and oldstable

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

Author: thaJeztah

.github/workflows/test.yml

@@ -17,7 +17,10 @@ jobs:
   build:
     strategy:
       matrix:
-        go-version: [1.21.x, 1.22.x]
+        go-version:
+          - 1.20.x      # oldest supported (see go.mod)
+          - oldstable
+          - stable
         os: [ubuntu-latest, windows-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     timeout-minutes: 10