Build: upgrade language to 1.3 in cli, compiler plugins and Gradle plugin

The language/API version restriction here is to make sure the code works
with Gradle of earlier versions, but all versions of Gradle that we
support already depend on Kotlin 1.3+.

libraries/tools/kotlin-gradle-plugin-api/build.gradle.kts

@@ -23,12 +23,12 @@ pill {
 
 tasks {
     withType<KotlinCompile> {
-        kotlinOptions.languageVersion = "1.2"
-        kotlinOptions.apiVersion = "1.2"
-        kotlinOptions.freeCompilerArgs += listOf("-Xskip-metadata-version-check")
+        kotlinOptions.languageVersion = "1.3"
+        kotlinOptions.apiVersion = "1.3"
+        kotlinOptions.freeCompilerArgs += listOf("-Xskip-prerelease-check")
     }
 
     named<Jar>("jar") {
         callGroovy("manifestAttributes", manifest, project)
     }
-}
+}