Remove migrations and add changelog for devs

[Chralt98]

What does it do?

It removes the pallet contracts removal migration and adds a changelog for the developers to inform about the 0.5.7 runtime upgrade change.

What important points should reviewers know?

Is there something left for follow-up PRs?

What alternative implementations were considered?

Are there relevant PRs or issues?

References

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.18%. Comparing base (940a8d8) to head (012a110).
Report is 5 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1405      +/-   ##
==========================================
- Coverage   93.19%   93.18%   -0.01%     
==========================================
  Files         131      131              
  Lines       29241    29274      +33     
==========================================
+ Hits        27252    27280      +28     
- Misses       1989     1994       +5     

Flag	Coverage Δ
tests	93.18% <ø> (-0.01%)	⬇️

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[Chralt98]

This PR needs to be merged after the #1398 merge.