build(deps): bump actions/cache from 5 to 6 (#33)

Author: dependabot[bot]

action.yml

@@ -134,7 +134,7 @@ runs:
     - name: Cache executables
       if: inputs.cache == 'true'
       id: cache-modflow
-      uses: actions/cache@v5
+      uses: actions/cache@v6
       with:
         path: ${{ env.MODFLOW_BIN_PATH }}
         key: ${{ steps.cache-key.outputs.key }}