Use OPT_IN instead of EXPERIMENTAL in diagnostic names

compiler/testData/diagnostics/testsWithStdLib/experimental/experimentalIsNotEnabled.kt

@@ -1,7 +1,7 @@
 // FIR_IDENTICAL
 // FILE: api.kt
 
-@<!EXPERIMENTAL_IS_NOT_ENABLED!>RequiresOptIn<!>
+@<!OPT_IN_IS_NOT_ENABLED!>RequiresOptIn<!>
 @Retention(AnnotationRetention.BINARY)
 annotation class Marker
 
@@ -11,7 +11,7 @@ fun f() {}
 // FILE: usage.kt
 
 fun use1() {
-    <!EXPERIMENTAL_API_USAGE_ERROR!>f<!>()
+    <!OPT_IN_USAGE_ERROR!>f<!>()
 }
 
 @Marker
@@ -19,7 +19,7 @@ fun use2() {
     f()
 }
 
-@<!EXPERIMENTAL_IS_NOT_ENABLED!>OptIn<!>(Marker::class)
+@<!OPT_IN_IS_NOT_ENABLED!>OptIn<!>(Marker::class)
 fun use3() {
     f()
 }