[avalanche] Rename the orphan API accessors to immature

Summary:
Accessing orphans has been deprecated in favor or immature proofs, so this diff updates the API to reflect that change. More renaming will come in follow up diffs.

Depends on D11860.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11861