Align RP ID string types

`rp.id` in `PublicKeyCredentialCreationOptions` and `rpId` in
`PublicKeyCredentialRequestOptions` represent the same thing, but with
different types. The WG agreed on the 2024-05-15 that both should be
`USVString`. Strictly speaking this is a breaking change (changing a
type bound in input (contravariant) position to be more restrictive),
but in practice this shouldn't be able to break any applications since
then those credentials wouldn't have worked in `get()` anyway.

index.bs

@@ -2906,7 +2906,7 @@ value and terminate the operation.
     dictionary PublicKeyCredentialRequestOptionsJSON {
         required Base64URLString                                challenge;
         unsigned long                                           timeout;
-        DOMString                                               rpId;
+        USVString                                               rpId;
         sequence<PublicKeyCredentialDescriptorJSON>             allowCredentials = [];
         DOMString                                               userVerification = "preferred";
         sequence<DOMString>                                     hints = [];
@@ -3235,7 +3235,7 @@ The {{PublicKeyCredentialRpEntity}} dictionary is used to supply additional [=[R
 
 <xmp class="idl">
     dictionary PublicKeyCredentialRpEntity : PublicKeyCredentialEntity {
-        DOMString      id;
+        USVString      id;
     };
 </xmp>