Target `stable` and `oldstable` in main CI

[meatballhat]

The GitHub Actions configuration in the main branch should be modified to align with the v1-maint and v2-maint branches such that the Go versions targeted are literally stable and oldstable. This will allow the version matrix to remain automatically updated as new Go versions are released. If a situation arises where we want to target an older Go version, I would like to discuss via a separate issue 💖