[OAuth] Deprecations and Qt##7 removal markings

There are several candidates for Qt7 removal or refactoring, which cannot be done in a BC/SC manner.

Deprecate:

• QAbstractOAuth2 get() post() etc methods. No direct replacement in qtnetworkauth, instead
  document how to use access tokens in network requests. Note: this is distinct from deprecating
  the QAbstractOauth get() post() etc methods, as they are also used with OAuth1. With OAuth1
  deprecation isn't as straightforward.

Qt7: removal markings

• QAbstractOAuth get() post() etc. methods
• Remove the undocumented QOAuthOobReplyHandler class and reorganise the few functionalities it has somewhere else. Oob made sense with OAuth1, and originally with OAuth2 (but not anymore).
• OAuth1 support. OAuth1 was spec-wise obsoleted in 2012 by OAuth2. By now, 12 years later, there's only very few OAuth1-only providers out there
• Simplify the inheritance hierarchy which should be possible when dropping the OAuth1 support