PYTHON-5609 Add 4.15.3 release to changelog

[blink1073]

Issue Key

Summary

Add the changelog entry from the 4.15.3 release.

Changes in this PR

Adds the changelog entry.

Testing Plan

Docs test that are run as part of the PR.

Screenshots (optional)

N/A

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is the intention of the code captured in relevant tests?
• If there are new TODOs, has a related JIRA ticket been created?

Checklist for Reviewer {@primary_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?)
• Have you checked for spelling & grammar errors?
• Is all relevant documentation (README or docstring) updated?

Focus Areas for Reviewer (optional)

[caseyclements]

LGTM