feat: set promotion shard in webhook

[krancour]

Fixes #1630

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 43.95%. Comparing base (f2149ef) to head (ec98337).

Files	Patch %	Lines
internal/webhook/promotion/webhook.go	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1633      +/-   ##
==========================================
- Coverage   43.96%   43.95%   -0.02%     
==========================================
  Files         199      199              
  Lines       12673    12674       +1     
==========================================
- Hits         5572     5571       -1     
- Misses       6864     6866       +2     
  Partials      237      237              

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