Bump actions/cache from 3 to 4 (#150)

Bumps [actions/cache](https://github.com/actions/cache) from 3 to 4.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](actions/cache@v3...v4)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/pytest.yaml

@@ -44,7 +44,7 @@ jobs:
           curl https://raw.githubusercontent.com/mne-tools/mne-testing-data/master/version.txt -o mne_testing_data_version.txt
       - name: Cache testing dataset
         id: cache
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           key: mne-testing-${{ runner.os }}-${{ hashFiles('mne_testing_data_version.txt') }}
           path: ~/mne_data
@@ -99,7 +99,7 @@ jobs:
           curl https://raw.githubusercontent.com/mne-tools/mne-testing-data/master/version.txt -o mne_testing_data_version.txt
       - name: Cache testing dataset
         id: cache
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           key: mne-testing-${{ runner.os }}-${{ hashFiles('mne_testing_data_version.txt') }}
           path: ~/mne_data