admin/on_call: Convert Event::send() to async fn

[Turbo87]

One less thing using reqwest::blocking 🎉

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 28 lines in your changes missing coverage. Please review.

Project coverage is 88.92%. Comparing base (429e1be) to head (9e8eb30).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/admin/test_pagerduty.rs	0.00%	14 Missing ⚠️
src/bin/monitor.rs	0.00%	10 Missing ⚠️
src/admin/on_call.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9581      +/-   ##
==========================================
+ Coverage   88.90%   88.92%   +0.02%     
==========================================
  Files         289      289              
  Lines       29123    29117       -6     
==========================================
+ Hits        25892    25893       +1     
+ Misses       3231     3224       -7     

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