Migrate Gradle deprecations (#1745)

pkl-tools/pkl-tools.gradle.kts

@@ -25,9 +25,9 @@ plugins {
   signing
 }
 
-val placeholder: SourceSet by sourceSets.creating
+val placeholder = sourceSets.create("placeholder")
 
-val firstPartySourcesJars by configurations.existing
+val firstPartySourcesJars = configurations.named("firstPartySourcesJars")
 
 // Note: pkl-tools cannot (easily) contain pkl-config-kotlin
 // because pkl-tools ships with a shaded Kotlin stdlib.
@@ -55,12 +55,13 @@ dependencies {
 
 // TODO: need to figure out how to properly generate javadoc here.
 // For now, we'll include a placeholder javadoc jar.
-val javadocPlaceholder by tasks.registering(Javadoc::class) { source = placeholder.allJava }
+val javadocPlaceholder =
+  tasks.register<Javadoc>("javadocPlaceholder") { source = placeholder.allJava }
 
 java { withJavadocJar() }
 
-val javadocJar by
-  tasks.existing(Jar::class) {
+val javadocJar =
+  tasks.named<Jar>("javadocJar") {
     from(javadocPlaceholder)
     archiveBaseName.set("pkl-tools-all")
     archiveClassifier.set("javadoc")
@@ -105,8 +106,8 @@ private fun Exec.configureTestStartFatJar(launcher: Provider<JavaLauncher>) {
   }
 }
 
-val testStartFatJar by
-  tasks.registering(Exec::class) { configureTestStartFatJar(buildInfo.javaTestLauncher) }
+val testStartFatJar =
+  tasks.register<Exec>("testStartFatJar") { configureTestStartFatJar(buildInfo.javaTestLauncher) }
 
 tasks.validateFatJar { dependsOn(testStartFatJar) }