Quiesce warnings from grep in sync-version

[akx]

Summary

(?:...) non-capturing groups are a PCRE thing, not a grep ERE thing.

As we can see in e.g. GitHub Actions runs, sync-version as of current main throws six (count 'em!) warnings when run. (Looks like this warning was added into GNU tools some 4 years ago.)

Run scripts/check
+ ./scripts/sync-version
grep: warning: ? at start of expression
grep: warning: ? at start of expression
grep: warning: ? at start of expression
grep: warning: ? at start of expression
grep: warning: ? at start of expression
grep: warning: ? at start of expression
CHANGELOG_VERSION: 0.28.1
VERSION: 0.28.1

This removes the ?:, so no more warnings, still the same grep power.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
  • No tests required.
• I've updated the documentation accordingly.
  • No documentation updates required.