Upgrade JVM toolchain to 25 and Kotlin toolchain to 2.3.20 (#1516)

Motivation
- Enable correct NullAway analysis
- Pick up toolchain fixes and improvements

Toolchains
- Require JDK 25 for JVM toolchain (keep Java 17 runtime compatibility)
- Require Kotlin 2.3.20 for Kotlin toolchain (keep Kotlin 2.2 runtime
compatibility)
- Require JDK 25 for Gradle daemon JVM (via
gradle-daemon-jvm.properties)
- Fix javac and kotlinc warnings from toolchain upgrades

CI
- Bump GitHub workflows to JDK 25

Building Kotlin
- Bump Kotlin language level to 2.2 to match stdlib version
- Consolidate build logic into pklKotlinBase.gradle.kts
- Adopt modern Kotlin plugin syntax
- Fix new kotlinc warnings
- Update ktfmt to 0.62
  - first version compatible with Kotlin 2.3.20
  - changes formatting compared to 0.61
- Replace dependency resolution rule with BOM alignment
  - rule was too broad and interfered with toolchain/runtime separation

Testing
- Expand matrix to JDK 25 (LTS) and 26
- Ensure each matrix task can be run independently
- Fix KotlinCodeGeneratorsTest and EmbeddedExecutorsTest on affected
JDKs
- Disable one test in CliCommandTest on affected JDKs (failure cause
unknown)

Compatibility fixes
- Fix reflective access in DocGenerator on affected JDKs

Build fixes
- Fix misuse of `task.enabled` vs. `report.required`
- Fix `gradlew tasks` on Windows
- Downgrade Spotless to 8.3.0 to (hopefully) work around sporadic
NoClassDefFoundError

Result
- NullAway runs correctly
- Broader JDK test coverage
- More reproducible and potentially faster builds

pkl-gradle/src/test/kotlin/org/pkl/gradle/KotlinCodeGeneratorsTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -108,18 +108,24 @@ class KotlinCodeGeneratorsTest : AbstractTest() {
   }
 
   private fun writeBuildFile() {
-    val kotlinVersion = "2.0.21"
+    val jvmTargetVersion = "17"
+    val kotlinToolchainVersion = "2.3.20"
+    val kotlinTargetVersion = "2.2"
+    val kotlinStdLibVersion = "2.2.21"
 
     writeFile(
       "build.gradle",
       """
+      import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+      import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
+      
       buildscript {
         repositories {
           mavenCentral()
         }
 
         dependencies {
-          classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion") {
+          classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinToolchainVersion") {
             exclude module: "kotlin-android-extensions"
           }
         }
@@ -131,12 +137,23 @@ class KotlinCodeGeneratorsTest : AbstractTest() {
 
       apply plugin: "kotlin"
 
+      tasks.withType(JavaCompile).configureEach {
+        options.release = $jvmTargetVersion
+      }
+      
+      kotlin {
+        compilerOptions {
+          jvmTarget = JvmTarget.fromTarget($jvmTargetVersion.toString())
+          languageVersion = KotlinVersion.fromVersion($kotlinTargetVersion.toString())
+        }
+      }
+      
       repositories {
         mavenCentral()
       }
 
       dependencies {
-        implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlinVersion"
+        implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlinStdLibVersion"
       }
 
       pkl {