tool-versions: Revert upgrade to Go 1.21.2
This reverts merge request !6455 (merged), which has upgraded the Go version to v1.21.2. This has caused downstream breakage when building GDK images because they do not yet support that Go version.
This reverts merge request !6455 (merged), which has upgraded the Go version to v1.21.2. This has caused downstream breakage when building GDK images because they do not yet support that Go version.