[#5024] Remove uppercase validation of Domain in ZGW registration backend

[alextreme]

Closes #5024

Changes

Removing validation of Domain in ZGW registration backend

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Dockerfile/scripts

  • Updated the Dockerfile with the necessary scripts from the ./bin folder

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.72%. Comparing base (3f79ba2) to head (a6e3f2f).
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5075      +/-   ##
==========================================
- Coverage   96.72%   96.72%   -0.01%     
==========================================
  Files         770      770              
  Lines       26517    26516       -1     
  Branches     3454     3454              
==========================================
- Hits        25649    25648       -1     
  Misses        606      606              
  Partials      262      262              

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[sergei-maertens]

Checked with Alex and decided not to backport it because migrations complicate this process. It's okay if this only lands in 3.1.0