Rollout of GHA-based build workflows

This issue aggregates the different items involved in rolling out Github Actions as the default provider for builds.

- [X] Disable special casing in conda-smithy and make GHA a default provider
- [X] https://github.com/conda-forge/conda-smithy/pull/2478
- [X] https://github.com/conda-forge/conda-smithy/pull/2485
- [X] https://github.com/conda-forge/conda-smithy-feedstock/pull/375
- [X] https://github.com/conda-forge/conda-forge.github.io/pull/2768
- [X] The default had to be [rolled back](https://github.com/conda-forge/conda-forge.github.io/pull/2770) to Azure in https://github.com/conda-forge/conda-smithy/pull/2488, released in [3.56.2](https://github.com/conda-forge/conda-smithy-feedstock/pull/376) due to:
- [x] Figure out how to roll out GHA workflows upon rerenders from forks ([#general > conda-smithy rerender for 3.56.1 resulting in broken builds](https://conda-forge.zulipchat.com/#narrow/channel/457337-general/topic/conda-smithy.20rerender.20for.203.2E56.2E1.20resulting.20in.20broken.20builds/with/578391097))
- [X] https://github.com/conda-forge/conda-smithy/pull/2495
- [X] https://github.com/conda-forge/conda-smithy/pull/2498
- [X] https://github.com/conda-forge/conda-smithy/pull/2499
- [X] https://github.com/conda-forge/conda-smithy/releases/tag/v3.57.0
- [X] https://github.com/conda-forge/conda-smithy-feedstock/pull/378
- [x] End admin migration (https://github.com/conda-forge/admin-migrations/pull/178)
- [X] Fix mixing self-hosted and Github-hosted runners for GHA
- [X] https://github.com/conda-forge/conda-smithy-feedstock/pull/379
- [X] https://github.com/conda-forge/conda-smithy-feedstock/pull/380
- [ ] macOS concurrency is very limited: https://github.com/conda-forge/conda-smithy/issues/2486
- [ ] Re-enable macOS on GHA
- [x] Feature parity on Windows with Azure workflows: https://github.com/conda-forge/conda-smithy/issues/2349
- [ ] Re-enable Windows on GHA: https://github.com/conda-forge/conda-smithy/pull/2601

Optional:

- [x] https://github.com/conda-forge/conda-forge.github.io/issues/2852
- [x] https://github.com/conda-forge/conda-forge-bot/pull/6240