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.

buildSrc/src/main/kotlin/pklPublishLibrary.gradle.kts

@@ -54,7 +54,7 @@ val validatePom by tasks.registering {
     return@registering
   }
   val generatePomFileForLibraryPublication by tasks.existing(GenerateMavenPom::class)
-  val outputFile = file("$buildDir/validatePom") // dummy output to satisfy up-to-date check
+  val outputFile = file("build/validatePom") // dummy output to satisfy up-to-date check
 
   dependsOn(generatePomFileForLibraryPublication)
   inputs.file(generatePomFileForLibraryPublication.get().destination)