Use filesMatchpattern instead of includesModifiedFiles

[gewarren]

If a PR edits a disallowed file and an allowed file, it doesn't seem to trigger the policy (example). So try this instead.

[learn-build-service-prod[bot]]

Learn Build status updates of commit a4f2562:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
.github/policies/disallow-edits.yml	:white_check_mark:Succeeded

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