GitHub Actions: improve selected list of Go versions

- reverse list of Go versions to put the most recent at the top to get
  results earlier
- use symbolic names 'stable', 'oldstable' for the 2 latest
- add missing Go 1.15

.github/workflows/go.yml

@@ -11,16 +11,17 @@ jobs:
     strategy:
       matrix:
         go:
-          - '1.12'
-          - '1.13'
-          - '1.14'
-          - '1.16'
-          - '1.17'
-          - '1.18'
-          - '1.19'
+          - 'stable'
+          - 'oldstable'
           - '1.20'
-          - '1.21'
-          - '1.22'
+          - '1.19'
+          - '1.18'
+          - '1.17'
+          - '1.16'
+          - '1.15'
+          - '1.14'
+          - '1.13'
+          - '1.12'
 
     steps:
     - name: Check out code