Update pin for libunwind

[regro-cf-autotick-bot]

This PR has been triggered in an effort to update the pin for libunwind. The current pinned version is 1.6, the latest available version is 18 and the max pin pattern is x. This migration will impact 11 feedstocks.

Checklist:

• The new version is a stable supported pin.
• I checked that the ABI changed from 1.6 to 18.

**Please note that if you close this PR we presume that the new pin has been rejected.

@conda-forge-admin please ping libcxx
This PR was generated by - please use this URL for debugging.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-webservice.

I was asked to ping @conda-forge/libcxx and so here I am doing that.

[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]

Ugh, sorry about this one. This is bogus, as a result of conda-forge/libcxx-feedstock#155. Not sure why the bot thinks this is available, as the builds for libunwind never got uploaded (cc @beckermr)

I did open conda-forge/libunwind-feedstock#22 though, which would be a legitimate bump.

[h-vetinari]

We should probably wait for conda-forge/libcxx-feedstock#160 before merging, so the bot doesn't keep misdetecting the version and reopening PRs.

[h-vetinari]

Closing this in favour of #6215