Add checksum for Gradle wrapper (#538)

The verification fails on `main`, even though the run in #535 was green.
Probably because the action version had to be updated as well.

Author: TimvdLippe

.github/workflows/ci.yml

@@ -37,7 +37,7 @@ jobs:
           java-version: 11
 
       - name: 3. Validate Gradle wrapper
-        uses: gradle/wrapper-validation-action@v1 # https://github.com/gradle/wrapper-validation-action
+        uses: gradle/actions/wrapper-validation@v4 # https://github.com/gradle/wrapper-validation-action
 
   #
   # Main build job

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,6 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
+distributionSha256Sum=20f1b1176237254a6fc204d8434196fa11a4cfb387567519c61556e8710aed78
 distributionUrl=https\://services.gradle.org/distributions/gradle-8.13-bin.zip
 networkTimeout=10000
 zipStoreBase=GRADLE_USER_HOME