Remove pygments pin

[calumy]

Summary

The recent release of Pygments (2.19.1) allows the pinned version to be removed as the PYI alias for Python syntax highlighting has been removed.

Test Plan

• Follow the steps outlined in https://github.com/astral-sh/ruff/blob/main/CONTRIBUTING.md#mkdocs to get the documentation site running locally.
• Spot test rules pages that have PYI code blocks to ensure that syntax highlighting remains e.g. http://127.0.0.1:8000/ruff/rules/complex-if-statement-in-stub/.

Note: I am unable to test the insiders build but would assume that it functions locally as I do not have access to MkDocs Insiders, but I would like to assume that it functions in the same way as the non-insiders build.