ci: bump actions/cache from 3 to 4

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-version: '4'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/docs.yml

@@ -17,7 +17,7 @@ jobs:
         with:
           python-version: "3.12"
 
-      - uses: actions/cache@v3
+      - uses: actions/cache@v4
         with:
           path: ~/.cache/pip
           key: ${{ runner.os }}-pip-docs-${{ hashFiles('docs/requirements.txt') }}

.github/workflows/doctest_validation.yml

@@ -27,7 +27,7 @@ jobs:
         python-version: ${{ matrix.python-version }}
 
     - name: Cache conda environment
-      uses: actions/cache@v3
+      uses: actions/cache@v4
       with:
         path: ~/conda_pkgs_dir
         key: ${{ runner.os }}-conda-${{ hashFiles('solarwindpy-20250403.yml') }}
@@ -210,7 +210,7 @@ jobs:
         python-version: ${{ matrix.python-version }}
 
     - name: Cache conda environment
-      uses: actions/cache@v3
+      uses: actions/cache@v4
       with:
         path: ~/conda_pkgs_dir
         key: ${{ runner.os }}-conda-${{ hashFiles('solarwindpy-20250403.yml') }}

.github/workflows/sync-requirements.yml

@@ -28,7 +28,7 @@ jobs:
           python-version: "3.11"
 
       - name: Cache pip packages
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: ~/.cache/pip
           key: ${{ runner.os }}-pip-sync-${{ hashFiles('requirements-dev.txt') }}