PYTHON-6153 Remove redundant exclude_newer: "" from uv-lock-update.yml

[blink1073]

PYTHON-6153

Changes in this PR

• Removes the workflow's exclude_newer override, made redundant by PYTHON-6152 removing the input from the action; the repository's own uv config now solely decides the cutoff.
• Keeps the lock-update cutoff as exclude-newer = "7 days" in pyproject.toml.
• Will bump the action pin to the new release once PYTHON-6152 merges and tags.

Test Plan

• Workflow-only change, so no unit tests; validated by YAML parse and a clean grep for the removed input.
• Safe in either merge order: a no-op against the current pin, a warning-only no-op against the new release.
• A post-merge workflow_dispatch dry run exercises the workflow end to end.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)? Not needed; workflow-only change.
• Is there test coverage? No unit tests apply; validation strategy in Test Plan.
• 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?