[FE] Replace some MUST_BE_INITIALIZED messages with MUST_BE_INITIALIZED_OR_BE_FINAL

From user point of view it's an improvement in compilation message.

From technical point of view it's an introduction of new compilation
diagnostic.

Review: https://jetbrains.team/p/kt/reviews/9967

I'm going to deprecate `open val` case in the next few commits KT-57553.
But it is always possible to suggest using `final` for `open val` case.

compiler/testData/diagnostics/tests/backingField/MustBeInitializedEffectivelyFinalOff.kt

@@ -5,7 +5,7 @@ open class Base {
 }
 
 class Foo : Base() {
-    <!MUST_BE_INITIALIZED_OR_BE_ABSTRACT!>override var x: String<!>
+    <!MUST_BE_INITIALIZED_OR_FINAL_OR_ABSTRACT!>override var x: String<!>
 
     init {
         x = ""