refactor: fetch root key method names

[ghost]

Description

Renames (with deprecation0 two methods, since the criterion involved is whether the network is mainnet or not, rather than whether it is the local network or not, and the "local" setting also applies to testnets.

• fetch_root_key_when_local -> fetch_root_key_when_non_mainnet
• fetch_root_key_when_local_or_error -> fetch_root_key_when_non_mainnet_or_error

The motivation is an upcoming improvement to the interface used at https://github.com/dfinity/dfx-extensions/blob/main/extensions/nns/src/commands/install.rs#L70

How Has This Been Tested?

Refactor covered by CI

Checklist:

• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.