RFR: 8331346: Update PreviewFeature of STREAM_GATHERERS to JEP-473

[Viktor Klang]

This PR finalizes JEP 473 by modifying the PreviewFeature JEP number and status for Stream Gatherers.

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Commit messages:
 - Updating PreviewFeature.STREAM_GATHERERS to 473 - Second Preview

Changes: https://git.openjdk.org/jdk/pull/19003/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19003&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331346
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19003.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19003/head:pull/19003

PR: https://git.openjdk.org/jdk/pull/19003