close various open migrations for abseil/grpc/protobuf

[h-vetinari]

Now that #5829 was merged, we can remove the old migrators that were needed to keep some feedstocks already on a newer deployment target.

The usual set of suspects hasn't completed the last grpc/protobuf migration yet, but since it's been open for two month, there's not much more to do but move on.

For reference here are the PRs still open according to the status page:

• Rebuild for libprotobuf 4.25.3 mongodb-feedstock#91
• Rebuild for libprotobuf 4.25.3 or-tools-feedstock#56
• Rebuild for libprotobuf 4.25.3 pytorch-cpu-feedstock#228
• Rebuild for libprotobuf 4.25.3 xilinx-runtime-feedstock#32

Dead feedstocks:

• Rebuild for libprotobuf 4.25.3 bear-feedstock#45
• https://github.com/conda-forge/go-cockroach-feedstock
• https://github.com/conda-forge/tomviz-feedstock

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[h-vetinari]

CC @conda-forge/abseil-cpp @conda-forge/grpc-cpp @conda-forge/libprotobuf @conda-forge/protobuf