Merge #19174: refactor: replace CConnman pointers by references in net_processing.cpp

0c8461a refactor: replace CConnman pointers by references in net_processing.cpp (Sebastian Falbesoner)

Pull request description:

  This is a follow-up to the recently merged PR bitcoin/bitcoin#19053, replacing ~~two more types of~~ one more type of pointer (CConnman) by references to increase the code quality -- pointers should either check for `nullptr` or be replaced by references, and the latter strategy seems to be more reasonable.

  Again, to keep the review burden managable, the changes are kept simple,
  * only tackling `CConnman*` ~~and `BanMan*`~~ pointers
  * only within the net_processing module, i.e. no changes that would need adaption in other modules
  * keeping the names of the variables as they are

ACKs for top commit:
  jnewbery:
    utACK 0c8461a
  MarcoFalke:
    ACK 0c8461a 🕧

Tree-SHA512: 79dc05144bcfb5e0bbc62180285aadcc6199f044fa3016c0f54f7b7f45037415260970037bd63b18fafefb8aef448549dae14b780bafb540fa2373f493a17f71

Author: MarcoFalke