update GHAs, clean up yml, use micromamba

[ocefpaf]

In this PR:

1. update the GitHub Actions versions
2. move from conda to micromamba to get faster builds
3. clean up the plugin tests a bit (we created a matrix when we did not really needed it)
4. run the tests on master, main and develop (we should probably get rid of master BTW)

This is not ready for review yet. I'll work on caching the environment first to make it easier to re-run tests in a PR.

[codecov]

Codecov Report

Merging #932 (6565548) into master (54f7644) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #932   +/-   ##
=======================================
  Coverage   80.32%   80.32%           
=======================================
  Files          23       23           
  Lines        5102     5102           
  Branches     1273     1273           
=======================================
  Hits         4098     4098           
  Misses        714      714           
  Partials      290      290           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 54f7644...6565548. Read the comment docs.

[benjwadams]

Closing due to merge of #980

[ocefpaf]

Done in #980

[ocefpaf]

Closing due to merge of #980

We probably clicked almost at the same time :-)