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+.

compiler/cli/build.gradle.kts

@@ -55,9 +55,9 @@ sourceSets {
 
 tasks.withType<org.jetbrains.kotlin.gradle.dsl.KotlinCompile<*>> {
     kotlinOptions {
-        languageVersion = "1.2"
-        apiVersion = "1.2"
-        freeCompilerArgs += "-Xskip-metadata-version-check"
+        languageVersion = "1.3"
+        apiVersion = "1.3"
+        freeCompilerArgs += "-Xskip-prerelease-check"
     }
 }