[wip] Set Connection: close header when draining HTTP/1

[dprotaso]

Follow up to #3295

This will trigger the http server to close the connection https://cs.opensource.google/go/go/+/refs/tags/go1.25.4:src/net/http/server.go;l=1389-1391

I'm going to hold this PR and see if we need it

[dprotaso]

/hold

[knative-prow]

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[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.57%. Comparing base (e853b1d) to head (33177e0).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
network/handlers/drain.go	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3297      +/-   ##
==========================================
- Coverage   74.58%   74.57%   -0.01%     
==========================================
  Files         188      188              
  Lines        8187     8185       -2     
==========================================
- Hits         6106     6104       -2     
  Misses       1841     1841              
  Partials      240      240              

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[dprotaso]

Closing this out - I haven't seen webhook EOFs since I removed the disabling of keep alives