Update release procedure & compatibility tests

[glatterf42]

While working on iiasa/message_ix#919, I noticed two things that were outdated/missing in our docs, so this PR includes them:

1. The release procedure did not mention conda-forge. It now does, but we don't have any test to check if new releases can be installed via conda properly. We should probably set up a test analogous to message_ix. If we want to do that, I'll set up a new PR for it.
2. Our front README page mentions versions we test for compatibility, but it has fallen out of date. This PR updates them, please make sure it is now correct/accurate (e.g. based on the CI tests triggered for this PR).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.0%. Comparing base (c7ea1f3) to head (015520d).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #165   +/-   ##
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  Coverage   87.0%   87.0%           
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  Files        230     230           
  Lines       8170    8170           
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  Hits        7112    7112           
  Misses      1058    1058