* Upgrade Go to 1.21
* ci: detect Go backwards compatibility test version automatically
For our submodules and other places we choose to test against previous
Go versions, detect this version automatically from the current one
rather than hard-coding it.
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.
Deniz Onur Duzgun
skpratt
Michael Zalimeni