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-codegen-java/src/main/kotlin/org/pkl/codegen/java/JavaCodeGenerator.kt

@@ -82,7 +82,7 @@ data class JavaCodeGeneratorOptions(
    *
    * The specified annotation type must have a [java.lang.annotation.Target] of
    * [java.lang.annotation.ElementType.TYPE_USE] or the generated code may not compile. If set to
-   * `null`, [org.pkl.config.java.mapper.NonNull] will be used.
+   * `null`, `org.pkl.config.java.mapper.NonNull` will be used.
    */
   val nonNullAnnotation: String? = null,
 
@@ -105,7 +105,7 @@ class JavaCodeGenerator(
 ) {
 
   companion object {
-    private val OBJECT = ClassName.get(Object::class.java)
+    private val OBJECT = ClassName.get(Any::class.java)
     private val STRING = ClassName.get(String::class.java)
     private val DURATION = ClassName.get(Duration::class.java)
     private val DURATION_UNIT = ClassName.get(DurationUnit::class.java)
@@ -295,7 +295,7 @@ class JavaCodeGenerator(
         MethodSpec.methodBuilder("equals")
           .addModifiers(Modifier.PUBLIC)
           .addAnnotation(Override::class.java)
-          .addParameter(Object::class.java, "obj")
+          .addParameter(Any::class.java, "obj")
           .returns(Boolean::class.java)
           .addStatement("if (this == obj) return true")
           .addStatement("if (obj == null) return false")
@@ -418,7 +418,7 @@ class JavaCodeGenerator(
       val hasJavadoc =
         docComment != null && codegenOptions.generateJavadoc && !codegenOptions.generateGetters
       if (hasJavadoc) {
-        builder.addJavadoc(renderAsJavadoc(docComment!!))
+        builder.addJavadoc(renderAsJavadoc(docComment))
       }
 
       if (codegenOptions.generateGetters) {
@@ -466,7 +466,7 @@ class JavaCodeGenerator(
       val docComment = property.docComment
       val hasJavadoc = docComment != null && codegenOptions.generateJavadoc
       if (hasJavadoc) {
-        builder.addJavadoc(renderAsJavadoc(docComment!!))
+        builder.addJavadoc(renderAsJavadoc(docComment))
       }
 
       generateDeprecation(
@@ -583,7 +583,7 @@ class JavaCodeGenerator(
       val docComment = pClass.docComment
       val hasJavadoc = docComment != null && codegenOptions.generateJavadoc
       if (hasJavadoc) {
-        builder.addJavadoc(renderAsJavadoc(docComment!!))
+        builder.addJavadoc(renderAsJavadoc(docComment))
       }
 
       generateDeprecation(
@@ -707,7 +707,7 @@ class JavaCodeGenerator(
       .addModifiers(Modifier.STATIC)
       .addParameter(StringBuilder::class.java, "builder")
       .addParameter(String::class.java, "name")
-      .addParameter(Object::class.java, "value")
+      .addParameter(Any::class.java, "value")
       .addStatement("builder.append(\"\\n  \").append(name).append(\" = \")")
       .addStatement(
         "\$T lines = \$T.toString(value).split(\"\\n\")",