Add 1.78 to migrations (as it is replacing 1.74)

[jakirkham]

There is an active Boost migration to 1.78, which other migrations (like Python 3.11) need to work with. Have added this branch based on the last state of 1.78 in this feedstock (as main is on 1.80). Should ensure we start getting migration PRs there.

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Checklist

• [ ] Used a personal fork of the feedstock to propose changes
• [ ] Bumped the build number (if the version is unchanged)
• [ ] Reset the build number to 0 (if the version changed)
• [ ] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [ ] Ensured the license file is being packaged.

[conda-forge-linter]

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.

[jakirkham]

No need to wait for CI. Merging so we can pick up a Python 3.11 migration PR for 1.78 as well.