Bump actions/cache from 5 to 6

Bumps [actions/cache](https://github.com/actions/cache) from 5 to 6.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](actions/cache@v5...v6)

---
updated-dependencies:
- dependency-name: actions/cache
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/pdf-at-pr.yaml

@@ -32,7 +32,7 @@ jobs:
           python-version: 3.8
 
       - name: Cache pip dependencies
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: ~/.cache/pip
           key: ${{ runner.os }}-pip-${{ hashFiles('documentation/requirements.txt') }}