Drop old versions (#979)

Author: mattn

.github/workflows/go.yaml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest]
-        go: ['1.12.17', '1.13.15', '1.14.7', '1.15', '1.16', '1.17']
+        go: ['1.15', '1.16', '1.17']
       fail-fast: false
     env:
       OS: ${{ matrix.os }}