add compatibility note about pre-final specifications

[Sakurann]

Multiple non-final specifications are referenced in OpenID4VP. we should add a note about their versions.

Th precedent is OpenID Connect Core final placing such compatibility note in the spec at https://openid.net/specs/openid-connect-core-1_0-final.html#PreFinalIETFSpecs :

Implementers should be aware that this specification uses several IETF specifications that are not yet final specifications. Those specifications are:

JSON Web Token (JWT) draft -13 [JWT]
JSON Web Signature (JWS) draft -18 [JWS]
JSON Web Encryption (JWE) draft -18 [JWE]
JSON Web Key (JWK) draft -18 [JWK]
JSON Web Algorithms draft -18 [JWA]
Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants draft -13 [OAuth.Assertions]
JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication and Authorization Grants draft -07 [OAuth.JWT]

While every effort will be made to prevent breaking changes to these specifications, should they occur, OpenID Connect implementations should continue to use the specifically referenced draft versions above in preference to the final versions, unless using a possible future OpenID Connect profile or specification that updates some or all of these references.