Move migrations in sub-directories

[martinboulais]

I DON'T have JIRA ticket

• explain what this PR does
• if it is a new feature, explain how you plan to use it
• tests are added
• documentation was updated or added

Notable changes for developers:

• Migrations are now grouped by release to have less files in one directory

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.10%. Comparing base (c923f23) to head (1ba606e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1758      +/-   ##
==========================================
+ Coverage   43.38%   44.10%   +0.71%     
==========================================
  Files         882      982     +100     
  Lines       15497    16242     +745     
  Branches     2895     2901       +6     
==========================================
+ Hits         6724     7164     +440     
- Misses       8773     9078     +305     

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