request_uri_method should be case sensitive

[jogu]

https://openid.net/specs/openid-4-verifiable-presentations-1_0.html#section-5.1 currently says:

request_uri_method: OPTIONAL. A string determining the HTTP method to be used when the request_uri parameter is included in the same request. Two case-sensitive valid values are defined in this specification

I'm dubious that it makes sense to have a parameter where some values can be case sensitive and some case insensitive. I think we should just define it as a case sensitive string.