RFR: 8342707: Prepare Gatherers for graduation from Preview

[Viktor Klang]

Make final adjustments to drop PreviewFeature and updating the @ since markers.

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Commit messages:
 - Removing PreviewFeature from Gatherers
 - Updating @since to 24 for Gatherers

Changes: https://git.openjdk.org/jdk/pull/21686/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21686&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342707
  Stats: 35 lines in 24 files changed: 0 ins; 17 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/21686.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21686/head:pull/21686

PR: https://git.openjdk.org/jdk/pull/21686