[avalanche] Update the getrawavalancheproof RPC to return 'immature' instead of 'orphan'
Summary:
If the proof has immature utxos, then flag it as immature instead of orphan.
More renaming will occur in follow up diffs.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, sdulfari
Reviewed By: #bitcoin_abc, sdulfari
Differential Revision: https://reviews.bitcoinabc.org/D11860