PYTHON-6072 - Update changelog for 4.18 release

[blink1073]

PYTHON-6072

Changes in this PR

Sets the PyMongo 4.18.0 release date and finalizes the version for release.

• Updated the 4.18.0 changelog header date from 2026/XX/XX to 2026/09/03.
• Bumped pymongo/_version.py from 4.18.0.dev0 to 4.18.0.

Test Plan

No functional code changed. pre-commit hooks (ruff, ruff-format, synchro, rstcheck) passed on commit.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage? Docs/version-only change; no code to test.
• Is any followup work tracked in a JIRA ticket? If so, add link(s). Followup: cut the v4.18 release branch and publish 4.18.0, tracked in PYTHON-6072.

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?

[Copilot]

🟢 Approval recommended

The changes are limited to release metadata (version string and changelog date) and are internally consistent with no functional impact.

Pull request overview

This PR finalizes the PyMongo 4.18.0 release metadata by setting the release date in the changelog and switching the library version from a development suffix to the final release version.

Changes:

• Set the 4.18.0 changelog header date to 2026/09/03.
• Bumped __version__ from 4.18.0.dev0 to 4.18.0.

File summaries

File	Description
pymongo/_version.py	Updates the reported PyMongo version to the final 4.18.0 release string.
doc/changelog.rst	Finalizes the 4.18.0 changelog header date for the release.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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