RFR: 8336914: Shenandoah: Missing verification steps after JDK-8255765

[duke]

On Fri, 26 Jul 2024 21:20:27 GMT, Satyen Subramaniam <duke at openjdk.org> wrote:

> Adding before-update-refs verification step which was removed in previous revision [JDK-8255765](https://bugs.openjdk.org/browse/JDK-8255765) as directed by [JDK-8336914](https://bugs.openjdk.org/browse/JDK-8336914)

@satyenme 
Your change (at version 56ddb4a5c670bf7eeb4fa4f2436412ddb57a371c) is now ready to be sponsored by a Committer.

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PR Comment: https://git.openjdk.org/jdk/pull/20364#issuecomment-2291699568