RFR: 8347617: Shenandoah: Use consistent name for update references phase

[Y. Srinivas Ramakrishna]

On Thu, 16 Jan 2025 19:43:35 GMT, William Kemper <wkemper at openjdk.org> wrote:

> During the review of https://github.com/openjdk/jdk/pull/22688, we discovered that some methods and variables used `updaterefs`, while most used `update_refs` (or `UpdateRefs`). This PR changes all of the `updaterefs` names to `update_refs` for consistency.

Thanks for making these mutually consistent!

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

Marked as reviewed by ysr (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23164#pullrequestreview-2557180908