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

[William Kemper]

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.

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

Commit messages:
 - Use consistent names for update references

Changes: https://git.openjdk.org/jdk/pull/23164/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23164&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347617
  Stats: 64 lines in 18 files changed: 0 ins; 0 del; 64 mod
  Patch: https://git.openjdk.org/jdk/pull/23164.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23164/head:pull/23164

PR: https://git.openjdk.org/jdk/pull/23164