TODOs for next build

Since rebuilding takes so much effort and we don't do it that often, I wanted to create an issue to track the various things we should adapt for the next build:

EDIT: "checked" below means it's currently included in #176
- [x] bump to [tensorflow 2.7.0](https://github.com/tensorflow/tensorflow/issues/50204)
- [x] needs keras-builds: https://github.com/conda-forge/keras-feedstock/pull/56
- [ ] needs packaging tensorflow-io-gcs-filesystem: [PyPI](https://pypi.org/project/tensorflow-io-gcs-filesystem/) [github](https://github.com/tensorflow/io)
- [ ] remove bazel-workarounds that should now be taken care of in `bazel-toolchain` (see https://github.com/conda-forge/bazel-toolchain-feedstock/pull/5)
- [x] next gprc migration:
- https://github.com/conda-forge/tensorflow-feedstock/pull/177
- [x] Add `cudatoolkit` as a dependency to `tensorflow`, or even better, get rid of `tensorflow-base` (since it's not needed anymore now that `tf-estimator` is built in this feedstock), see https://github.com/conda-forge/tensorflow-feedstock/issues/162
- note that "getting rid" means in terms of the core recipe, there could/should still be a compatibility output for existing users in that case.
- [x] Loosen package pinnings where possible, see #151
- [x] In particular, `numpy` has been unpinned from 1.19.2 in TF 2.7 as of https://github.com/tensorflow/tensorflow/commit/a90383a867a2337600c775d67842f308da9d118b
- [x] Add a run-depend on `__cuda` virtual package or the GPU builds, see #174
- [x] Python 3.10 builds? The migrator is currently failing to resolve the builds, probably not least due to the tight package pinnings (and there being no numpy 1.19 for cpython 3.10).
- [x] Remove `gilbertfrancois` from the maintainers: https://github.com/conda-forge/tensorflow-feedstock/issues/178
- [ ] Unskip `libtensorflow{,_cc}`, see #181.

CC @hmaarrfk @xhochy @wolfv @conda-forge/tensorflow, feel free to add more things in the description. If any of those points end up being a huge hassle, it can of course be pushed off to the build after that.

Anyone is welcome to chime in if I forgot something. 🙃