[FIXED JENKINS-26601] Partly reverting #1453 to avoid hitting https://github.com/google/guice/issues/899.

(cherry picked from commit 00c858d4 into 1.599 rc)

In this environment, Util.createSymlink and .isSymlink are implemented, but resolveSymlink is not.
(cherry picked from commit 388c4b5e)

Conflicts:
	changelog.html

In this environment, Util.createSymlink and .isSymlink are implemented, but resolveSymlink is not.

[FIXED JENKINS-26601] Partly reverting #1453 to avoid hitting https://github.com/google/guice/issues/899.

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