Remove pygments pin (#15404)
## Summary
The recent release of Pygments
([2.19.1](https://github.com/pygments/pygments/releases/tag/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/](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.
