[aclark4life]

PYTHON-6155

Changes in this PR

Prepare the 4.18.3 patch release from v4.18.

• Set __version__ to 4.18.3 in pymongo/_version.py.
• Add the 4.18.3 changelog entry, linking the 4.18.3 Jira release notes.

Test Plan

Pre-commit hooks pass, including synchro. The release workflow consumes
pymongo/_version.py to tag the release.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?