chore(deps): Bump kotlin language + plugin version

The newer Kotlin Gradle plugin no longer supports maintaining our existing
language version target. To keep our build toolchain up-to-date, we've had to
bump the target language version to the minimum supported by the newest Kotlin
plugin. We expect these sorts of bumps to continue for the forseeable future
because of overly tight coupling between the Kotlin gradle plugin and the
Kotlin compiler.

This may cause compatibility breaks for some users. Those users should consider
using a programming language that cares about stability if this is an issue.

extra-kotlin/build.gradle.kts

@@ -12,7 +12,7 @@ kotlin {
   compilerOptions {
     jvmTarget = JvmTarget.JVM_1_8
     @Suppress("DEPRECATION")
-    languageVersion = KotlinVersion.KOTLIN_1_4
+    languageVersion = KotlinVersion.KOTLIN_1_6
   }
 }
 

gradle/libs.versions.toml

@@ -75,5 +75,5 @@ zJmh = { module = "org.openjdk.jmh:jmh-core", version.ref = "jmh" }
 [plugins]
 indra-sonatype = { id = "net.kyori.indra.publishing.sonatype", version.ref = "indra" }
 jmh = { id = "me.champeau.jmh", version.ref = "jmhPlugin" }
-kotlin = "org.jetbrains.kotlin.jvm:2.0.21"
+kotlin = "org.jetbrains.kotlin.jvm:2.1.0"
 nexusPublish = { id = "io.github.gradle-nexus.publish-plugin", version = "2.0.0" }

renovate.json

@@ -4,8 +4,7 @@
   ],
   "ignoreDeps": [
     "com.google.code.gson:gson",
-    "org.slf4j:slf4j-api",
-    "org.jetbrains.kotlin.jvm"
+    "org.slf4j:slf4j-api"
   ],
   "packageRules": [
     {