Updates to CI and README

[duckontheweb]

Description:

• Removes Gitter badge since that channel is mostly unused
• Adds orjson extra install instructions
• Minor clarifications around versions
• Only run release build for this repo (I was seeing failed builds in my fork when I updated tags)
• Move coverage reporting into separate job
  Since we added the minimum coverage requirement, the test job fails before uploading coverage to Codecov.io. When I was working on removing support for pre-0.8 STAC versions I found myself wishing I could use that UI to figure out where the coverage had dropped. This change means the CI workflow will still fail overall, but we will still get a coverage report we can look at if we drop below the minimum threshhold.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #452 (6174b4c) into main (a729cd2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #452   +/-   ##
=======================================
  Coverage   91.31%   91.31%           
=======================================
  Files          40       40           
  Lines        5204     5204           
=======================================
  Hits         4752     4752           
  Misses        452      452           

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[lossyrob]

+1 with one suggestion