RFR: 8365939: [Redo] G1: Move collection set related full gc reset code into abandon_collection_set() method

[Thomas Schatzl]

Hi all,

  please review this attempt on clearing the young regions cset group early in `abandon_collection_set()` during full gc.

The initial attempt failed because some verification code depended on the young gen remembered set being `Complete` and contain valid card entries.

This change fixes both:

- we need to clear the `young_regions_cset_group` with `uninstall_group_cardsets` being `true` to a) remove the cards and b) make it incomplete
- some verification expects that the young gen remset is always complete, which is not necessary during full gc.

Test: tier1-5, the failing test that caused the failures in JDK-8365780 passes always.

Thanks,
  Thomas

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Commit messages:
 - 8365939

Changes: https://git.openjdk.org/jdk/pull/26948/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26948&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365939
  Stats: 10 lines in 3 files changed: 4 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26948.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26948/head:pull/26948

PR: https://git.openjdk.org/jdk/pull/26948