Add a comment about Mutation Events VTS, and push out expiration

Since this virtual suite needs to stay well after the feature has
been disabled, I added a comment with the "why". This also pushes
out the expiration date to the M135 stable date.

Bug: 40268638
Change-Id: Ic537c0ae418b812338ebd7182cb08169fd823ccb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5314893


Commit-Queue: Di Zhang <dizhangg@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: Di Zhang <dizhangg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279995}