add bits related to conda-forge package: badge, links, Github Actions **and** unpin mamba-version from all GHA flows

[valeriupredoi]

Description

We now have a nice and shiny conda-forge package, so am adding things related to it.

Relates to #231

Also, I closed conda-incubator/setup-miniconda#392 since the default mamba is now 2.1.1 - a much later version; as such, all GHA files have had mamba-version: 2.0.5 removed.

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• If you are working on the documentation, please ensure the current build passes
• All tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.20%. Comparing base (3265118) to head (bf2e5df).
⚠️ Report is 48 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #278   +/-   ##
=======================================
  Coverage   85.20%   85.20%           
=======================================
  Files           7        7           
  Lines         649      649           
=======================================
  Hits          553      553           
  Misses         96       96           

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