cmd/go: change `go mod init` default go directive back to 1.N [1.26 backport]

[gopherbot]

@aclements requested issue #77653 to be considered for backport to the next 1.26 minor release.

@gopherbot , please backport to Go 1.26.

[aclements]

#77653 is the proposal to revert this change in behavior, introduced in Go 1.26.0. That proposal is currently in "likely accept" and is expected to move to "accept" next Wednesday. Since we're aiming for next week's 1.26.1 release, let's go ahead and prepare the backport CL, but DO NOT land it until #77653 is accepted.

[dmitshur]

Added a release-blocker label to indicate the release shouldn't happen while this issue is still open.

[gopherbot]

Change https://go.dev/cl/749948 mentions this issue: [release-branch.go1.26] cmd/go: revert update default go directive in mod or work init