The problem is, when you modified the go version in go.mod, the behaviors of some code change, but the change is not easy to detect in time. Go team never plan to develop a tool to identify/detect code affected by such breaking changes, leaving developers without guarantees for safe migration.
And consider that some people run go scripts even without the "//go:build go1.xx" directives ... (Please don't refute me. The Go toolchain allows this and never warns on this.)