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/build.gradle.kts

@@ -1,3 +1,5 @@
+import org.jetbrains.kotlin.config.JvmTarget
+
 plugins {
   `kotlin-dsl`
 }
@@ -19,3 +21,13 @@ java {
   sourceCompatibility = JavaVersion.VERSION_11
   targetCompatibility = JavaVersion.VERSION_11
 }
+
+kotlin {
+  target {
+    compilations.configureEach {
+      kotlinOptions {
+        jvmTarget = "11"
+      }
+    }
+  }
+}