Add a kapt language version override (#920)

This allows us to force it to a lower version if needed for K2 testing

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slack-plugin/src/main/kotlin/slack/gradle/SlackProperties.kt

@@ -19,6 +19,7 @@ import java.io.File
 import java.util.Locale
 import org.gradle.api.Project
 import org.gradle.api.provider.Provider
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 import slack.gradle.anvil.AnvilMode
 import slack.gradle.artifacts.SgpArtifact
 import slack.gradle.util.PropertyResolver
@@ -618,6 +619,13 @@ internal constructor(
         AnvilMode.valueOf(it.uppercase(Locale.US))
       }
 
+  /** Overrides the kotlin language version if present. */
+  public val kaptLanguageVersion: Provider<KotlinVersion>
+    get() =
+      resolver.optionalStringProvider("sgp.kapt.languageVersion").map {
+        KotlinVersion.fromVersion(it)
+      }
+
   /** Defines a required vendor for JDK toolchains. */
   public val jvmVendor: Provider<String>
     get() =

slack-plugin/src/main/kotlin/slack/gradle/kgp/KgpTasks.kt

@@ -68,12 +68,28 @@ internal object KgpTasks {
         val isKaptGenerateStubsTask = this is KaptGenerateStubsTask
 
         compilerOptions {
-          progressiveMode.set(slackProperties.kotlinProgressive)
+          if (isKaptGenerateStubsTask && slackProperties.kaptLanguageVersion.isPresent) {
+            val zipped =
+              slackProperties.kotlinProgressive.zip(slackProperties.kaptLanguageVersion) {
+                progressive,
+                kaptLanguageVersion ->
+                if (kaptLanguageVersion != KotlinVersion.DEFAULT) {
+                  false
+                } else {
+                  progressive
+                }
+              }
+            progressiveMode.set(zipped)
+          } else {
+            progressiveMode.set(slackProperties.kotlinProgressive)
+          }
           optIn.addAll(slackProperties.kotlinOptIn)
           if (slackProperties.kotlinLanguageVersionOverride.isPresent) {
             languageVersion.set(
               slackProperties.kotlinLanguageVersionOverride.map(KotlinVersion::fromVersion)
             )
+          } else if (isKaptGenerateStubsTask && slackProperties.kaptLanguageVersion.isPresent) {
+            languageVersion.set(slackProperties.kaptLanguageVersion)
           }
           if (
             !slackProperties.allowWarnings &&