* build: consolidate Envoy version management
Simplify Envoy version management by consolidating all runtime, build,
and CI sources of Envoy versions into a single plaintext file.
The goal of this change is to avoid common mistakes missing an update of
some Envoy versions (both in general and due to release branch
inconsistency), and enable automated Envoy version updates in the
future.
* ci: add missing ref argument for get-go-version
Supports nightly tests.
* 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.
Michael Zalimeni
Dan Stough