fix(self-hosted): Fix type of retention days

[untitaker]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.94%. Comparing base (ada1f34) to head (1f8c37a).
Report is 7 commits behind head on master.

Files	Patch %	Lines
snuba/settings/settings_self_hosted.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5591   +/-   ##
=======================================
  Coverage   89.93%   89.94%           
=======================================
  Files         897      900    +3     
  Lines       43538    43624   +86     
  Branches      288      288           
=======================================
+ Hits        39155    39236   +81     
- Misses       4341     4346    +5     
  Partials       42       42           

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