For #36460, Go 1.17 will add redundancy in go.mod files that would be removed by go mod tidy in earlier versions of the toolchain. We should make go mod tidy fail if it doesn't definitively know what invariants a tidy module requires — that is, if the go.mod file specifies a newer-than-supported version of the toolchain.

(@jayconrod, @matloob: I thought we already had an issue filed for this but I can't find it now. Does it ring a bell?)