[FE] Add clear warning about future changes about nullability of safe call with non nullable receiver

^KT-46860

compiler/testData/diagnostics/tests/SafeCallInvoke.fir.kt

@@ -18,6 +18,6 @@ fun foo() {
     // these both also ok (with smart cast / unnecessary safe call)
     if (rule != null) {
         rule.apply()
-        rule<!UNNECESSARY_SAFE_CALL!>?.<!>apply()
+        <!SAFE_CALL_WILL_CHANGE_NULLABILITY!>rule<!UNNECESSARY_SAFE_CALL!>?.<!>apply()<!>
     }
 }