Starting in Go 1.21, Go uses a new version format for toolchains which uses the 1.N.P syntax. This is a problem for our users when their go.mod files only contain a go directive that uses the old version format, e.g. go 1.22. 1.22 is a valid language version, but not a valid toolchain version. If there is no additional toolchain directive in the go.mod file, then go uses the language version as the toolchain version. In the example, 1.22 is not a valid toolchain version and certain go commands, particularly those which try to acquire the toolchain corresponding to that version, fail with an unhelpful error.

In this PR, we make two changes to the Go autobuilder:

1. We detect go.mod files where the language version is 1.21 or greater, doesn't use the new toolchain version syntax, and there is no explicit toolchain directive. We emit a warning-level diagnostic in this case.
2. We explicitly invoke go version with GOTOOLCHAIN=go1.N.P (i.e. the correct representation of the toolchain version) to get go to install that version and avoid subsequent errors despite the incorrect version format. As an optimisation for repos with a lot of go.mod files, we now keep track of toolchain versions that we know are installed and only do this if we don't yet know that the corresponding version is installed.