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

buildSrc/src/main/kotlin/pklKotlinBase.gradle.kts

@@ -0,0 +1,67 @@
+/*
+ * Copyright © 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.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+import org.gradle.accessors.dm.LibrariesForLibs
+import org.gradle.kotlin.dsl.getByType
+import org.gradle.kotlin.dsl.kotlin
+import org.gradle.kotlin.dsl.the
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
+
+plugins {
+  java
+  kotlin("jvm")
+  id("com.diffplug.spotless")
+}
+
+val buildInfo = project.extensions.getByType<BuildInfo>()
+
+val libs = the<LibrariesForLibs>()
+
+dependencies {
+  // Align versions of Kotlin modules during dependency resolution.
+  // Do NOT align "api", as this would affect consumers' builds.
+  implementation(platform(libs.kotlinBom))
+  testImplementation(platform(libs.kotlinBom))
+}
+
+kotlin {
+  compilerOptions {
+    val kotlinTarget = KotlinVersion.fromVersion(libs.versions.kotlinTarget.get())
+    languageVersion.set(kotlinTarget)
+    apiVersion.set(kotlinTarget)
+    jvmTarget = JvmTarget.fromTarget(buildInfo.jvmTarget.toString())
+    jvmToolchain {
+      languageVersion.set(buildInfo.jdkToolchainVersion)
+      vendor.set(buildInfo.jdkVendor)
+    }
+    freeCompilerArgs.addAll(
+      "-jvm-default=no-compatibility", // was: -Xjvm-default=all
+      "-Xjdk-release=${buildInfo.jvmTarget}",
+      "-Xjsr305=strict",
+    )
+  }
+}
+
+spotless {
+  val revertYearOnlyChanges = RevertYearOnlyChangesStep(rootProject.rootDir, ratchetFrom!!).create()
+
+  kotlin {
+    addStep(revertYearOnlyChanges)
+    ktfmt(libs.versions.ktfmt.get()).googleStyle()
+    target("src/*/kotlin/**/*.kt")
+    licenseHeaderFile(rootProject.file("buildSrc/src/main/resources/license-header.star-block.txt"))
+  }
+}