add new abseil/grpc/protobuf migration

[h-vetinari]

Add a new migration, paused for now, while we prepare things (getting all interrelated versions ready, plus bazel).

I would have like to include grpc 1.70, but they've had a massive regression in terms of how many symbols get put into grpc.dll (factor 8 more than before), and despite spending a lot of effort to split off something and get below that limit in for 1.68, they're right back over the limit again. 😑

This probably needs a bisect...

Closes #7082
Closes #7083
Closes #7089
Closes #7097
Closes #7101

[conda-forge-admin]

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/meta.yaml) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe/meta.yaml:

• ℹ️ top-level output has some malformed specs:

• In section test: python >= 3.8 Requirements spec fields should always be space-separated to avoid known issues in conda-build. For example, instead of name =version=build, use name version build`.

_{This message was generated by GitHub Actions workflow run https://github.com/conda-forge/conda-forge-webservices/actions/runs/13662141146. Examine the logs at this URL for more detail.}

[conda-forge-admin]

Hi! This is the friendly conda-forge automerge bot!

I considered the following status checks when analyzing this PR:

• linter: passed
• github-actions: passed

Thus the PR was passing and merged! Have a great day!