Fix multi-jdk testing setup (#1086)

This changes the Gradle build to always create multi-jdk tasks,
and instead use the `enabled` property to determine whether the task
is actually ran or not.

The has the following benefits:
* IntelliJ and other tools understand the task execution graph (e.g. testJdk20 shows up as a task)
* JDK-specific Gradle configurations always exist, so `updateDependencyLocks` is consistent.

pkl-tools/pkl-tools.gradle.kts

@@ -120,6 +120,7 @@ for (jdkTarget in buildInfo.jdkTestRange) {
   } else {
     val task =
       tasks.register("testStartFatJarJdk${jdkTarget.asInt()}", Exec::class) {
+        enabled = buildInfo.isVersionEnabled(jdkTarget)
         val launcher = project.javaToolchains.launcherFor { languageVersion = jdkTarget }
         configureTestStartFatJar(launcher)
       }