RFR: 8320913: GenShen: Bootstrap 21u backports repo

[Aleksey Shipilev]

On Mon, 27 Nov 2023 20:18:52 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Merges the generational mode for Shenandoah to a jdk21u based repo.

> /sponsor needs to be done by @shipilev or @rkennke?

LOL, this is the first time I see bot misinterprets the command like this. 🔥 
Another thing we should try some time:

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PR Comment: https://git.openjdk.org/shenandoah-jdk21u/pull/1#issuecomment-1832481806