ci: delete Go 1.15 and add Go 1.18 (#6847)

.github/workflows/go.yml

@@ -54,7 +54,7 @@ jobs:
     name: Test
     strategy:
       matrix:
-        go-version: [ 1.15.x, 1.16.x, 1.17.x ]
+        go-version: [ 1.16.x, 1.17.x, 1.18.x ]
         platform: [ ubuntu-latest, macos-latest, windows-latest ]
     runs-on: ${{ matrix.platform }}
     steps: