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/build-docs.yml

@@ -30,7 +30,7 @@ jobs:
         run: poetry config virtualenvs.in-project true
 
       - name: Set up cache
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         id: cache
         with:
           path: |

.github/workflows/contrib-tests.yml

@@ -57,7 +57,7 @@ jobs:
           poetry-version: "2.3.1"
 
       - name: Set up cache
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         id: cache
         with:
           path: |

.github/workflows/python-tests.yml

@@ -54,7 +54,7 @@ jobs:
         run: poetry config virtualenvs.in-project true
 
       - name: Set up cache
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         id: cache
         with:
           path: |
@@ -107,7 +107,7 @@ jobs:
         run: poetry config virtualenvs.in-project true
 
       - name: Set up cache
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         id: cache
         with:
           path: |