RFR: 8341692: Implement JEP 490: ZGC: Remove the Non-Generational Mode

[Axel Boldt-Christmas]

This is the implementation task for `JEP 490: ZGC: Remove the Non-Generational Mode`. See the JEP for details. [JDK-8335850](https://bugs.openjdk.org/browse/JDK-8335850)

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Commit messages:
 - Remove XCollectedHeap from HSDB
 - Fix typo in TestZUncommitEvent.java
 - Add missing problem-listing
 - Remove x from jdk.hotspot.agent
 - 8341692: Implement JEP 490: ZGC: Remove the Non-Generational Mode

Changes: https://git.openjdk.org/jdk/pull/21401/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21401&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341692
  Stats: 39423 lines in 406 files changed: 152 ins; 39003 del; 268 mod
  Patch: https://git.openjdk.org/jdk/pull/21401.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21401/head:pull/21401

PR: https://git.openjdk.org/jdk/pull/21401