.github/workflows: update to Go 1.21

As our official policy is to support the latest version of Go, we
upgrade our CI workflow to version 1.21, which was recently released.

We also drop support for versions older than 1.20 as they are
not supported by upstream Go.

Author: chrisd8088

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     name: Default build
     strategy:
       matrix:
-        go: ['1.18.x', '1.19.x']
+        go: ['1.20.x', '1.21.x']
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4