ci: Set Go version consistently via .go-version
Ensure Go version is determined consistently for CI and Docker builds
rather than spread across several different files.
The intent is to eventually replace this with use of the `toolchain`
directive in Go 1.21.
Due to the unintuitive behavior of GHA w.r.t. implicit status check
`success()`, test results were only being uploaded on success (failures
presumably came from retried tests that passed).
Dan Stough
Michael Zalimeni
Manoj Srinivasamurthy
Nick Irvine