Upgrade actions/cache from v2 to v4

Author: kklamberty

.github/workflows/gtest.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Cache gtest library
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       env:
         cache-name: cache-gtest-lib
       with: