[backend] Fix schema of StixFile where obsContent was also in schema attributes (#5751)

[lndrtrbn]

Proposed changes

• Remove obsContent from schema attributes and keep it only in schema refs

Related issues

• CSV mapper configuration can mix up entity types fields #5751

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

In my opinion there is also an issue when we want to add Artifacts to a File but I don't think it should be fixed in this issue.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3dfddbf) 64.72% compared to head (98e541d) 64.72%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5758      +/-   ##
==========================================
- Coverage   64.72%   64.72%   -0.01%     
==========================================
  Files         516      516              
  Lines       60890    60889       -1     
  Branches     4856     4856              
==========================================
- Hits        39410    39409       -1     
  Misses      21480    21480              

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