disallow '\xff' character syntax

[StefanKarpinski]

In light of #24439 '\xff' may become a syntax for a Char that contains invalid UTF-8 data rather than being another way of writing '\uff' – i.e. the code point 255 (ÿ). This is already illegal in string literals where \u or \U is required for code points above 127, so we would just be bringing character syntax in sync with string syntax. Note that we once upon a time allowed "\xff" as a string literal with invalid UTF-8 in it, which is why it's not allowed in strings. It is, however, allowed in byte string, i.e. b"\xff" produces [0xff]. We could allow \xff in strings, however, and all need for b"\xff" would go away since you can just use normal strings instead of the byte form. That would bring strings and characters fully in sync.