Consolidate OpenJDK 8u repos into one

We are planning on consolidating the jdk8u HG repositories into one single repo (similar to how it was done in JDK 10).

Things that still need to be decided:

* Cutoff time - Proposed November 29th 2021
* New name for consolidated https://hg.openjdk.java.net/jdk8u/jdk8u - Proposed https://hg.openjdk.java.net/jdk8u/monojdk8u
* New name for consolidated https://hg.openjdk.java.net/jdk8u/jdk8u-dev - Proposed https://hg.openjdk.java.net/jdk8u/monojdk8u-dev

The last change before the consolidation needs to be made before the cutoff date and it needs to be the final tag change in each repository. At that time, I will finalize the conversion into the single repository, based on https://hg.openjdk.java.net/jdk8u/jdk8u/. The new single repository will then be posted for both jdk8u and jdk8u-dev in their new locations.

Ideally this should just take one day, but to plan for the worst, we should be prepared for a week of downtime worst case.

Discussion so far has been taking place here: https://mail.openjdk.java.net/pipermail/jdk8u-dev/2021-October/014316.html