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-core/src/main/java/org/pkl/core/externalreader/ExternalReaderProcess.java

@@ -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.
@@ -71,9 +71,9 @@ public interface ExternalReaderProcess extends AutoCloseable {
    *
    * <p>This method can be safely called multiple times. Subsequent calls have no effect.
    *
-   * @implNote Implementers should request a graceful termination by sending a {@link
-   *     ExternalReaderMessages.CloseExternalProcess CloseExternalProcess} message to the process
-   *     before terminating it forcibly.
+   * <p><b>Implementation Note:</b> Implementers should request a graceful termination by sending a
+   * {@link ExternalReaderMessages.CloseExternalProcess CloseExternalProcess} message to the process
+   * before terminating it forcibly.
    */
   @Override
   void close();