Go: run with minimum version, not maximum

[rarkins]

Describe the proposed change(s).

Due to a change in Go behavior (see golang/go#62409), Go 1.21 now "upgrades" the Go directive in go.mod if we run go x commands with a newer Go version than is already there.

We should instead try to use the exact version of Go to stop this "accidental" upgrading.

Example commit from reproduction: renovate-reproductions/24994@5ee6ce9 (#1)

[rarkins]

This is not as simple as it seems. When I ran that same PR with 1.20 it errored:

         "stderr": "github.com/tuihub/librarian/internal/lib/libmq imports\n\tgithub.com/ThreeDotsLabs/watermill imports\n\tlog/slog: package log/slog is not in GOROOT (/opt/containerbase/tools/golang/1.20.8/src/log/slog)\nnote: imported by a module that requires go 1.21\n",

@MuZhou233 maybe Go only upgraded to 1.21 because it needed to and that's why you couldn't easily reproduce either?

[rarkins]

I've found that Go does not update the Go mod directive unless it's necessary