[backend] added migration for missed linked-to refs

[JeremyCloarec]

Proposed changes

• added a new migration on linked-to ref, targeting the READ_INDEX_STIX_CYBER_OBSERVABLE_RELATIONSHIPS indexes this time

Related issues

• Some x_opencti_linked-to ref relations weren't cleaned properly, causing schema validations error on impacted entities #8022

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case (coverage and e2e)
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.73%. Comparing base (202d1de) to head (88266c6).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8100   +/-   ##
=======================================
  Coverage   67.72%   67.73%           
=======================================
  Files         572      572           
  Lines       71041    71041           
  Branches     6045     6046    +1     
=======================================
+ Hits        48115    48120    +5     
+ Misses      22926    22921    -5     

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