Explicitly use language version 1.9 for K2-unsupported parts

This commit explicitly sets LV 1.9 for all projects with disabled FIR
bootstrap, as well as for libraries, including Native stdlib.

Related to KT-59171

kotlin-native/runtime/build.gradle.kts

@@ -589,6 +589,8 @@ konanArtifacts {
                 "-opt-in=kotlin.contracts.ExperimentalContracts",
                 "-opt-in=kotlin.ExperimentalMultiplatform",
                 "-opt-in=kotlin.native.internal.InternalForKotlinNative",
+                "-language-version",
+                "1.9",
         )
 
         commonStdlibSrcDirs.forEach { commonSrcDir(it) }

repo/gradle-build-conventions/buildsrc-compat/src/main/kotlin/LibrariesCommon.kt

@@ -13,6 +13,7 @@ import org.gradle.api.plugins.BasePluginExtension
 import org.gradle.api.tasks.compile.JavaCompile
 import org.gradle.kotlin.dsl.*
 import org.gradle.process.CommandLineArgumentProvider
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 import org.jetbrains.kotlin.gradle.tasks.KotlinJvmCompile
 
@@ -71,8 +72,14 @@ fun Project.configureFrontendIr() = tasks.withType<KotlinJvmCompile>().configure
         if (project.kotlinBuildProperties.useFirForLibraries) {
             freeCompilerArgs.add("-Xuse-k2")
             allWarningsAsErrors.set(false)
-        } else if (project.kotlinBuildProperties.useFir) {
-            freeCompilerArgs.add("-Xskip-prerelease-check")
+        } else {
+            if (project.kotlinBuildProperties.useFir) {
+                freeCompilerArgs.add("-Xskip-prerelease-check")
+            }
+            if (languageVersion.get() >= KotlinVersion.KOTLIN_2_0) {
+                languageVersion.set(KotlinVersion.KOTLIN_1_9)
+                progressiveMode.set(false)
+            }
         }
 
         val renderDiagnosticNames by extra(project.kotlinBuildProperties.renderDiagnosticNames)

repo/gradle-build-conventions/buildsrc-compat/src/main/kotlin/common-configuration.gradle.kts

@@ -1,4 +1,5 @@
 import org.gradle.internal.deprecation.DeprecatableConfiguration
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 import org.jetbrains.kotlin.gradle.plugin.KotlinBasePluginWrapper
 
 // Contains common configuration that should be applied to all projects
@@ -196,6 +197,10 @@ fun Project.configureKotlinCompilationOptions() {
                         freeCompilerArgs += "-Xskip-prerelease-check"
                     }
                 }
+                if (KotlinVersion.DEFAULT >= KotlinVersion.KOTLIN_2_0 && project.path in projectsWithDisabledFirBootstrap) {
+                    languageVersion = "1.9"
+                    options.progressiveMode.set(false)
+                }
                 if (renderDiagnosticNames) {
                     freeCompilerArgs += "-Xrender-internal-diagnostic-names"
                 }