[backend] Fix search broken in kill chain phases and marking definition (#6308 & #6318)

[SarahBocognano]

Proposed changes

• Add missing attribute in search

In this PR I decided to join to issues as the problem was exactly the same for both

Related issues

• Can't search in Taxonomies > Kill chain phases #6308
• Marking Definition Search Broken #6318

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

All modified and coverable lines are covered by tests ✅

Project coverage is 66.96%. Comparing base (70fb123) to head (e41f6f9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6394      +/-   ##
==========================================
- Coverage   66.97%   66.96%   -0.01%     
==========================================
  Files         541      541              
  Lines       64557    64561       +4     
  Branches     5313     5313              
==========================================
+ Hits        43234    43236       +2     
- Misses      21323    21325       +2     

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