Create a linux-aarch64-only migrator

[jaimergp]

Description:

Adds a ArchMigrator subclass that only deals with linux-aarch64, ignoring ppc64le for the sake of the discussion in conda-forge/conda-forge.github.io#2781.

Checklist:

• Pydantic model updated or no update needed

Cross-refs, links to issues, etc:

• RFC: Drop ppc64le conda-forge/conda-forge.github.io#2781

[jaimergp]

FAILED tests/test_make_migrators.py::test_make_migrators_initialize_migrators - FileNotFoundError: [Errno 2] No such file or directory: '/home/runner/micromamba/envs/cf-scripts/share/conda-forge/migrations/linux-aarch64.txt'

I guess we need to create that file in conda-forge-pinning first.

[beckermr]

We're going to reissue a bunch of duplicate PRs. We can't do that.

[minrk]

I think #5862 accomplishes this with minimal disruption, leaving the existing migrator in place and removing ppc from future migrations.