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.

docs/requirements-insiders.txt

@@ -5,6 +5,3 @@ mkdocs-material @ git+ssh://git@github.com/astral-sh/mkdocs-material-insiders.gi
 mkdocs-redirects==1.2.2
 mdformat==0.7.21
 mdformat-mkdocs==4.1.1
-# Using a commit from pygments main branch so we get
-# https://github.com/pygments/pygments/pull/2773 before it's been released
-pygments @ git+https://github.com/pygments/pygments.git@67b460fdde6d9a00342b5102b37b3a8399f0e8ef

docs/requirements.txt

@@ -5,6 +5,3 @@ mkdocs-material==9.5.38
 mkdocs-redirects==1.2.2
 mdformat==0.7.21
 mdformat-mkdocs==4.1.1
-# Using a commit from pygments main branch so we get
-# https://github.com/pygments/pygments/pull/2773 before it's been released
-pygments @ git+https://github.com/pygments/pygments.git@67b460fdde6d9a00342b5102b37b3a8399f0e8ef