More specific STAC_IO deprecation warnings

[duckontheweb]

Related Issue(s):

• RFC: Remove deprecated STAC_IO class #431

Description:

Moved this out of #443 so we can integrate it separately. Also added a Deprecated section to the 1.0.0-beta.1 CHANGELOG section so that the deprecation is more obvious there.

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 #449 (f832683) into main (e7d4bdc) will increase coverage by 0.09%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main     #449      +/-   ##
==========================================
+ Coverage   90.67%   90.76%   +0.09%     
==========================================
  Files          40       40              
  Lines        5105     5113       +8     
==========================================
+ Hits         4629     4641      +12     
+ Misses        476      472       -4     

Impacted Files	Coverage Δ
pystac/stac_io.py	76.71% <90.90%> (+4.24%)	⬆️

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