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.

Fixing 9162c3cb

Not fully solved, since the scannedDirs field can still grow to be large, but at least clearing files/dirsNotIncluded.
Also imposing a 5s timeout on the scan regardless of file count, and defining a user-customizable bound.