Add Deprecation Marker to x5t Usage

[jzheaux]

Since x5t uses SHA-1, it may be appropriate to add a deprecation marker to JoseHeaderNames.X5T, similar to how Nimbus does the same with its JWSHeader.Builder and Spring Security does with MessageDigestPasswordEncoder.

[pat-mccusker]

Can I throw up a pr with the annotation and some javadoc or you looking to have some further discussion first?

[jzheaux]

Thanks for checking, @pat-mccusker. I was still mulling it over at the time I wrote the ticket and hadn't gotten back around to updating the title.

Thanks again for your PR.