Use Gradle 8 compatible API

In Web3JGradle plugin replace an API that was removed in Gradle 8 with a
Gradle 8 API and an appropriate fallback.

src/main/java/org/web3j/gradle/plugin/Web3jPlugin.java

@@ -184,6 +184,11 @@ protected File buildOutputDir(final SourceSet sourceSet) {
         final SoliditySourceSet soliditySourceSet =
                 (SoliditySourceSet) convention.getPlugins().get(SoliditySourceSet.NAME);
 
-        return soliditySourceSet.getSolidity().getOutputDir();
+        try {
+            return soliditySourceSet.getSolidity().getDestinationDirectory().getAsFile().get();
+        } catch (NoSuchMethodError ignored) {
+            //TODO delete this catch block and unwrap the try block after Gradle 8 migration
+            return soliditySourceSet.getSolidity().getOutputDir();
+        }
     }
 }