Fix #12459: Warn instead of error if TASTy is not in sync with classfile

This matches the behavior in Scala, which was changed in this pull request:
scala/scala3#9125

It was changed as a result of this issue:
scala/scala3#8839

The commit message from the Scala 3 fix is as follows:
"Only warn if TASTy is not in sync with classfile

If they are not in sync, we warn and suggest to clean.
We assume that the TASTy is up to date (arbitrary choise) and load it regardless.
This way we are resiliant to the failiure if the loaded class API did not change or
did not have an impact on the code we are compiling."

Author: jjudd

src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala

@@ -1265,7 +1265,7 @@ abstract class ClassfileParser(reader: ReusableInstance[ReusableDataReader]) {
         val expectedUUID = new UUID(reader.readUncompressedLong(), reader.readUncompressedLong())
         val tastyUUID = new TastyHeaderUnpickler(TASTYBytes).readHeader()
         if (expectedUUID != tastyUUID) {
-          reporter.error(NoPosition, s"Tasty UUID ($tastyUUID) file did not correspond the tasty UUID ($expectedUUID) declared in the classfile $file.")
+          reporter.warning(NoPosition, s"$file is out of sync with its TASTy file. Loaded TASTy file. Try cleaning the project to fix this issue")
         }
         TASTYBytes
       }