RFR: 8329342: GenShen: Synchronize shenandoah-jdk21u:master with shenandoah:master

[William Kemper]

On Fri, 29 Mar 2024 22:08:06 GMT, William Kemper <wkemper at openjdk.org> wrote:

> The delta for this fork has grown too great for individual backport PRs. This change synchronizes the `shenandoah-jdk21u` fork by essentially checking out the `shenandoah` directory from `shenandoah` and reverting the few things we can't take because of changes outside of `shenandoah`.

Thanks for the review. I'm going to integrate once the GHA tests look good.

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PR Comment: https://git.openjdk.org/shenandoah-jdk21u/pull/31#issuecomment-2027825033