deprecate Step.__call__

[braingram]

This PR deprecates Step.__call__.

spacetelescope/romancal#1499 and spacetelescope/jwst#8945 removed all uses of Step.__call__ in romancal and jwst.

Tasks

• update or add relevant tests
• update relevant docstrings and / or docs/ page
• Does this PR change any API used downstream? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • run regression tests with this branch installed ("git+https://github.com/<fork>/stpipe@<branch>")
    • jwst regression test
    • romancal regression test

news fragment change types...

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: fixes an issue
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.misc.rst: infrastructure or miscellaneous change

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.80%. Comparing base (fd68d87) to head (45d4cde).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/stpipe/step.py	50.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (fd68d87) and HEAD (45d4cde). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (fd68d87)	HEAD (45d4cde)
	3	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #204       +/-   ##
===========================================
- Coverage   94.57%   80.80%   -13.78%     
===========================================
  Files          37       37               
  Lines        3172     2849      -323     
===========================================
- Hits         3000     2302      -698     
- Misses        172      547      +375     

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