cmd/go: extended forwards compatibility for Go [1.20 backport]

[gopherbot]

@dmitshur requested issue #57001 to be considered for backport to the next 1.20 minor release.

@gopherbot Please consider a specific subset of this proposal for backport to Go 1.20 and 1.19. Specifically, https://go.dev/doc/toolchain currently documents that:

[...] Go 1.​21 changed the go line to be a mandatory requirement instead. This behavior is partly backported to earlier language versions: Go 1.19 releases starting at Go 1.19.11 and Go 1.20 releases starting at Go 1.20.6, refuse to load workspaces or modules declaring version Go 1.​21 or later.

This backport issue tracks backporting that refusal of loading workspaces or modules declaring version Go 1.​21 or later. (If it ends up happening in later minor versions, the wording on https://go.dev/doc/toolchain can be updated retroactively.)

[rsc]

Fixed by CL 518675.

[rsc]

Note that the issue description above is incorrect: The important thing is to refuse to load Go 1.22 or later, since that's where we expect the incompatibility to arise. Allowing these releases to run Go 1.21 code is fine.