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

[Per Minborg]

On Mon, 29 Apr 2024 16:42:05 GMT, Viktor Klang <vklang at openjdk.org> wrote:

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

LGTM

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Marked as reviewed by pminborg (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/19003#pullrequestreview-2030731354