Make sure that pinned versions receive migrations

[Tobias-Fischer]

Recently in #1957 and #2013 we have seen that migrations have landed in the respective feedstocks of a package, but for a newer version than the one pinned in https://github.com/conda-forge/conda-forge-pinning-feedstock/blob/master/recipe/conda_build_config.yaml

This then leads to incompatibilities. I propose to add abi migration branches for those crucial feedstocks so that we don't end up in such a situation (or go for some smarter solution ;)).

[beckermr]

As long as you or the maintainers are willing to maintain the branches on the feedstocks, then go for it.

Overall, the core team has generally rejected automatically making the ABI branches since that would put an extra burden on maintainers.