[avalanche] Complete renaming from orphan to immature where applicable

Summary:
This rename the remaining variables and comments where this is relevant, so the code and comments are in par with the behavior.

Depends on D11864.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11866