[release-1.12] [CVE-2023-44487] Disable http2 for webhooks

[knative-prow-robot]

This is an automated cherry-pick of #2871

Due to CVE-2023-44487 webhooks will use http1 by default, until the issue is fixed properly.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (d6ab729) 81.88% compared to head (ea1ca18) 81.86%.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-1.12    #2876      +/-   ##
================================================
- Coverage         81.88%   81.86%   -0.02%     
================================================
  Files               167      167              
  Lines             10225    10231       +6     
================================================
+ Hits               8373     8376       +3     
- Misses             1606     1608       +2     
- Partials            246      247       +1     

Files	Coverage Δ
webhook/webhook.go	82.27% <50.00%> (-1.28%)	⬇️

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[krsna-m]

/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: knative-prow-robot, kvmware

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[krsna-m]

/lgtm