AK+Everywhere: Rename String to DeprecatedString

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 :^)

Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.cpp

@@ -51,7 +51,7 @@ SignedBigInteger SignedBigInteger::from_base(u16 N, StringView str)
     return { move(unsigned_data), sign };
 }
 
-String SignedBigInteger::to_base(u16 N) const
+DeprecatedString SignedBigInteger::to_base(u16 N) const
 {
     StringBuilder builder;