Remove AbstractMigrationBuilder + Mark readonly properties + Cleanup

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.69%. Comparing base (0c5303a) to head (815536a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #311      +/-   ##
============================================
+ Coverage     95.14%   99.69%   +4.55%     
+ Complexity      323      299      -24     
============================================
  Files            21       20       -1     
  Lines          1029      985      -44     
============================================
+ Hits            979      982       +3     
+ Misses           50        3      -47     

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