RFR: 8320913: GenShen: Bootstrap 21u backports repo

[William Kemper]

Merges the generational mode for Shenandoah to a jdk21u based repo.

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Commit messages:
 - Remove corretto files
 - Add a generational mode to the Shenandoah garbage collector
 - 8314610: hotspot can't compile with the latest of gtest because of <iomanip>

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/1/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=1&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320913
  Stats: 21645 lines in 219 files changed: 19682 ins; 930 del; 1033 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/1.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/1/head:pull/1

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/1