Use stable and oldstable version aliases in test config.

Don't hardcode version numbers, just test the current stable release and
the previous stable release, which is all the Go team officially
supports.

.github/workflows/test.yml

@@ -19,9 +19,8 @@ jobs:
       fail-fast: false
       matrix:
         go:
-          - '1.18'
-          - '1.19'
-          - '1.20'
+          - stable
+          - oldstable
 
     steps:
       - uses: actions/checkout@v4