chore: bump GitHub Action versions

`actions/checkout@v2` and `actions/cache@v2` currently issue deprecation
warnings during execution about deprecated Node.js version. See also [1].

[1]: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

.github/workflows/maven-ci.yml

@@ -15,15 +15,15 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: "Checkout sources"
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
     - name: "Set up Java"
       uses: actions/setup-java@v4
       with:
         java-version: ${{ matrix.java }}
         distribution: "temurin"
         cache: 'maven'
     - name: "Get Maven dependencies from cache"
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: ~/.m2/repository
         key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
@@ -38,7 +38,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: "Checkout sources"
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
     - name: "Setup Java"
       uses: actions/setup-java@v4
       with:
@@ -48,7 +48,7 @@ jobs:
         server-username: OSSRH_USERNAME
         server-password: OSSRH_PASSWORD
     - name: "Get Maven dependencies from cache"
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: ~/.m2/repository
         key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}