Get rid of hard referencing to LanguageLevel entries

It should make easier to use different intellij-core versions
Some of them doesn't have JDK_15_PREVIEW while the other don't have
JDK_17, while we're ok with both of them since they support records and
sealed interfaces

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/compiler/JavaLanguageLevel.kt

@@ -10,5 +10,8 @@ import com.intellij.openapi.roots.LanguageLevelProjectExtension
 import com.intellij.pom.java.LanguageLevel
 
 fun Project.setupHighestLanguageLevel() {
-    LanguageLevelProjectExtension.getInstance(this).languageLevel = LanguageLevel.JDK_15_PREVIEW
+    LanguageLevelProjectExtension.getInstance(this).languageLevel =
+        LanguageLevel.values().firstOrNull { it.name == "JDK_17" }
+            ?: LanguageLevel.values().firstOrNull { it.name == "JDK_15_PREVIEW" }
+                    ?: LanguageLevel.JDK_X
 }