[frontend/backend] make unit mandatory for Retention Rules

[Archidoit]

Proposed changes

• make retention_unit mandatory in RetentionRule
• add a migration to set retention_unit to 'days' for retention rules with no unit

Related issues

https://github.com/OpenCTI-Platform/opencti/issues/8190

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 66.14%. Comparing base (a3c0864) to head (c2fb322). Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
...atform/opencti-graphql/src/domain/retentionRule.js	66.66%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8200      +/-   ##
==========================================
- Coverage   66.15%   66.14%   -0.01%     
==========================================
  Files         597      597              
  Lines       59927    59932       +5     
  Branches     6150     6153       +3     
==========================================
- Hits        39644    39642       -2     
- Misses      20283    20290       +7     

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