Update pygments requirement from ~=2.16 to ~=2.18

Updates the requirements on [pygments](https://github.com/pygments/pygments) to permit the latest version.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](https://github.com/pygments/pygments/compare/2.16.0...2.18.0)

---
updated-dependencies:
- dependency-name: pygments
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

docs/requirements.txt

@@ -25,7 +25,7 @@ mkdocs-autolinks-plugin
 jinja2~=3.0
 markdown~=3.2
 mkdocs-material-extensions~=1.3
-pygments~=2.16
+pygments~=2.18
 pymdown-extensions~=10.2
 
 # Requirements for plugins