[backend] fix reverse relation (#5836)

[marieflorescontact]

Proposed changes

• Relation between Case and task is reversed and wasn't called correctly in the back

Related issues

• issue/5836

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

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (9be46c3) 65.02% compared to head (c6a6bef) 66.19%.
Report is 8 commits behind head on master.

Files	Patch %	Lines
...rm/opencti-graphql/src/modules/task/task-domain.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5942      +/-   ##
==========================================
+ Coverage   65.02%   66.19%   +1.16%     
==========================================
  Files         528      534       +6     
  Lines       62005    62703     +698     
  Branches     4967     5677     +710     
==========================================
+ Hits        40318    41505    +1187     
+ Misses      21687    21198     -489     

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