RFR: Make generational mode experimental

[Kelvin Nilsen]

On Wed, 24 May 2023 17:09:52 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Setting `-XX:ShenandoahGCMode=generational` now requires `XX:+UnlockExperimentalVMOptions`.

Thanks.

-------------

Marked as reviewed by kdnilsen (Committer).

PR Review: https://git.openjdk.org/shenandoah/pull/283#pullrequestreview-1442424666