.github/workflows: update to latest Go versions

Since the Go language community only provides support for the latest
two versions of Go, we also only provide official support for those two
versions in our Git LFS projects.

We therefore update our CI workflow for this project to match the
two Go versions currently supported and in use in the GitHub Actions
workflows of our primary git-lfs/git-lfs project.

Author: chrisd8088

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ jobs:
     name: Default build
     strategy:
       matrix:
-        go: ['1.12.x', '1.13.x', '1.14.x']
+        go: ['1.18.x', '1.19.x']
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v1