Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Services/DHCPClient/DHCPv4.h

@@ -151,7 +151,7 @@ struct ParsedDHCPv4Options {
         return values;
     }
 
-    DeprecatedString to_string() const
+    DeprecatedString to_deprecated_string() const
     {
         StringBuilder builder;
         builder.append("DHCP Options ("sv);