RFR: Use active generation, rather than young exclusively when preparing for update refs

[Kelvin Nilsen]

On Wed, 9 Feb 2022 18:35:23 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Without this code, global collections will not function correctly.

Marked as reviewed by kdnilsen (Committer).

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PR: https://git.openjdk.java.net/shenandoah/pull/113