Update to Gradle 8.6 (#245)

- Fix and clean up the pkl-commons-test build script.
- Change tests to read test packages/certs directly from
  the file system instead of packaging and reading them
  from the class path.
- Update expected checksums of some test packages.
- Fix a conflict between Pkl's and Gradle's
  Kotlin libraries in the pkl-gradle project.
- Fix build deprecation warnings.
- Ensure Gradle distribution integrity with `distributionSha256Sum`.
- Manually verify integrity of Gradle wrapper added by this commit.

pkl-gradle/pkl-gradle.gradle.kts

@@ -1,5 +1,3 @@
-import org.jetbrains.kotlin.utils.keysToMap
-
 plugins {
   pklAllProjects
   pklJavaLibrary
@@ -34,6 +32,14 @@ dependencies {
   testImplementation(projects.pklCommonsTest)
 }
 
+sourceSets {
+  test {
+    // Remove Gradle distribution JARs from test compile classpath.
+    // This prevents a conflict between Gradle's and Pkl's Kotlin versions.
+    compileClasspath = compileClasspath.filter { !(it.path.contains("dists")) }
+  }
+}
+
 publishing {
   publications {
     withType<MavenPublication>().configureEach {