fix languageVersion doc (#5125)

Author: martinbonnin

libraries/apollo-compiler/src/main/kotlin/com/apollographql/apollo3/compiler/codegen/kotlin/model/ModelBuilder.kt

@@ -1,10 +1,8 @@
 package com.apollographql.apollo3.compiler.codegen.kotlin.model
 
-import com.apollographql.apollo3.compiler.TargetLanguage.KOTLIN_1_5
 import com.apollographql.apollo3.compiler.applyIf
 import com.apollographql.apollo3.compiler.codegen.CodegenLayout.Companion.upperCamelCaseIgnoringNonLetters
 import com.apollographql.apollo3.compiler.codegen.kotlin.KotlinContext
-import com.apollographql.apollo3.compiler.codegen.kotlin.KotlinMemberNames
 import com.apollographql.apollo3.compiler.codegen.kotlin.KotlinSymbols
 import com.apollographql.apollo3.compiler.codegen.kotlin.adapter.from
 import com.apollographql.apollo3.compiler.codegen.kotlin.helpers.makeDataClassFromProperties
@@ -81,7 +79,7 @@ internal class ModelBuilder(
     val typeSpecBuilder = if (isInterface) {
       TypeSpec.interfaceBuilder(simpleName)
           // All interfaces can be sealed except if implementations exist in different packages (not allowed in Kotlin)
-          .applyIf(context.isTargetLanguageVersionAtLeast(KOTLIN_1_5) && hasSubclassesInSamePackage) {
+          .applyIf(hasSubclassesInSamePackage) {
             addModifiers(KModifier.SEALED)
           }
           .addProperties(properties)

libraries/apollo-gradle-plugin-external/src/main/kotlin/com/apollographql/apollo3/gradle/api/Service.kt

@@ -390,16 +390,9 @@ interface Service {
    *
    * Only valid when [generateKotlinModels] is `true`.
    *
-   * Must be either "1.4" or "1.5".
+   * For now only "1.5" is supported. Other versions might be added later as the Kotlin language evolves.
    *
-   * Note: "1.4", while still supported, is not useful since Apollo Kotlin requires Kotlin 1.5+. It is considered deprecated.
-   *
-   * Using an higher languageVersion allows generated code to use more language features like
-   * sealed interfaces in Kotlin 1.5 for an example.
-   *
-   * See also https://kotlinlang.org/docs/gradle.html#attributes-common-to-jvm-and-js
-   *
-   * Default: use the version of the Kotlin plugin.
+   * Default: "1.5"
    */
   val languageVersion: Property<String>
 

libraries/apollo-gradle-plugin-external/src/main/kotlin/com/apollographql/apollo3/gradle/internal/KotlinPluginFacade.kt

@@ -25,7 +25,7 @@ fun getKotlinTargetLanguage(userSpecified: String?): TargetLanguage {
       // val majorMinor = project.getKotlinPluginVersion()!!.take(3)
       TargetLanguage.KOTLIN_1_5
     }
-    else -> error("Apollo: languageVersion '$userSpecified' is not supported, must be either '1.4' or '1.5'")
+    else -> error("Apollo: languageVersion '$userSpecified' is not supported, Apollo Kotlin always generate Kotlin '1.5' source files")
   }
 }