The mutex profile for contention on runtime-internal locks now correctly points
to the end of the critical section that caused the delay. This matches the
profile's behavior for contention on `sync.Mutex` values. The
`runtimecontentionstacks` setting for `GODEBUG`, which allowed opting in to the
unusual behavior of Go 1.22 through 1.24 for this part of the profile, is now
gone.