Gradle deprecation fixes & upgrades (#1594)

* Fix Gradle deprecations

By using test suites, we explicitely configure the relevant dependencies
on the test sourceset. This is not done by merely configuring the test task.

* Switch to maintained version of Shadow

* Update to Gradle 8.14.2

build.gradle.kts

@@ -20,11 +20,11 @@ subprojects {
         testImplementation(rootProject.libs.junit)
     }
 
-    tasks {
-        test {
-            useJUnitPlatform()
-            reports {
-                junitXml.required.set(true)
+    testing.suites.named<JvmTestSuite>("test") {
+        useJUnitJupiter()
+        targets.all {
+            testTask.configure {
+                reports.junitXml.required = true
             }
         }
     }