[FIR-TEST] Update testdata of old frontend tests

compiler/testData/diagnostics/testsWithStdLib/smartcasts/letAlwaysChangesToNotNull.fir.kt

@@ -3,5 +3,5 @@
 fun foo(y: String) {
     var x: String? = null
     y.let { x = it }
-    x.length // Smart cast is not possible
+    x.<!INAPPLICABLE_CANDIDATE!>length<!> // Smart cast is not possible
 }

compiler/testData/diagnostics/testsWithStdLib/tryCatch/correctSmartcasts.fir.kt

@@ -30,7 +30,7 @@ fun test2(s: String?) {
             requireNotNull(s)
             t2 = true
         }
-        t2.<!UNRESOLVED_REFERENCE!>not<!>()
+        t2.not()
         s.length
     }
 }

compiler/testData/diagnostics/testsWithStdLib/tryCatch/correctSmartcasts_after.fir.kt

@@ -31,7 +31,7 @@ fun test2(s: String?) {
             requireNotNull(s)
             t2 = true
         }
-        t2.<!UNRESOLVED_REFERENCE!>not<!>()
+        t2.not()
         s.length
     }
 }