Deprecate old address-related RPCs

[str4d]

• getnewaddress
  • UAs will generate P2PKH address components internally, from a key tree that is domain separated from any addresses that might be returned by this RPC.
• z_getnewaddress
  • It has the semantics that each call generates a new spend authority for a new logical pool of funds.
  • Its name indicates it fetches a new address rather than a new key, which complicates address diversification.
  • It allows selecting a specific pool, but for UAs we won't be exposing that.
• z_gettotalbalance
  • Deprecate the old fields (but keep them for now, including Orchard balances in "private")
  • API itself is fine (we will extend the dictionary output with new fields)

[str4d]

#5300 marks getnewaddress and z_getnewaddress as deprecated in their help text. We can close this issue once z_gettotalbalance is also updated, and the deprecation is added to the release notes (which we can do in #5419 once we're ready to merge it).