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/pklHtmlValidator.gradle.kts

@@ -33,7 +33,7 @@ dependencies {
 }
 
 val validateHtml by tasks.registering(JavaExec::class) {
-  val resultFile = file("$buildDir/validateHtml/result.txt")
+  val resultFile = file("build/validateHtml/result.txt")
   inputs.files(htmlValidator.sources)
   outputs.file(resultFile)