Fixed JENKINS-26312, JENKINS-26298 and JENKINS-26306

- Recorded the fix in changelog.
- Added a separate screen to show the unmigration instruction, which
  makes it easier for people to find it than search logs.

This should work across builds and even across jobs, and does not force us to load other builds.

Also fixes JENKINS-25498

[FIXED JENKINS-25788] RunMap.remove was always returning false, even when the Run was in fact removed.