Deprecate the address_type field of the getnewaddress RPC

Summary:
And clarify the documentation. This field is very misleading as it does exactly nothing in our codebase and is only a bitcoin compatibility field.
Since it's an optional parameter there is no need to create a -deprecated_rpc option here.

Depends on D17066.

Test Plan:

bitcoin-cli help getnewaddress

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17067