We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
In the long-term, we should probably have a way to signal decoding
failure. For now, it should suffice to at least not crash. This is
particularly relevant because apparently this can be triggered while
parsing a PEM certificate, which happens during every TLS connection.
Found by OSS Fuzz
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=38979
