fix: convert CharFields to TextFields for FeatureImport / FeatureExport models

[matthewelwell]

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Changes

To support Oracle and MySQL (in our enterprise edition), we need to convert the CharFields here to TextFields, since the underlying column types in these database don't support the size we need.

How did you test this code?

Ran the migration and confirmed that no data was lost.

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[github-actions]

Uffizzi Preview deployment-49518 was deleted.

[zachaysan]

Looks good, although the constants could have been used for the migration.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.89%. Comparing base (d5f76ff) to head (a6e99f0).
Report is 666 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3720   +/-   ##
=======================================
  Coverage   95.89%   95.89%           
=======================================
  Files        1101     1102    +1     
  Lines       34681    34686    +5     
=======================================
+ Hits        33256    33261    +5     
  Misses       1425     1425           

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