Switch Webhook to ed25519 and expiration of the certificate to seven days

[Harwayne]

Changes

• 🧹 Webhook uses ed25519 rather than RSA 2048.
• 🧹 Change the expiration of the certificate to seven days and the recreation time to one day before expiration.

/kind enhancement

Release Note

NONE

Docs

[codecov]

Codecov Report

Merging #1998 (fcb4b4a) into master (32a3248) will decrease coverage by 0.04%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##           master    #1998      +/-   ##
==========================================
- Coverage   67.29%   67.25%   -0.05%     
==========================================
  Files         214      214              
  Lines        9061     9065       +4     
==========================================
- Hits         6098     6097       -1     
- Misses       2693     2696       +3     
- Partials      270      272       +2     

Impacted Files	Coverage Δ
webhook/certificates/certificates.go	69.69% <0.00%> (ø)
webhook/certificates/resources/certs.go	60.71% <72.72%> (-1.79%)	⬇️
webhook/certificates/resources/secret.go	85.71% <100.00%> (ø)
test/gcs/mock/mock.go	91.39% <0.00%> (-1.08%)	⬇️

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[dprotaso]

cc @markusthoemmes @julz are security focused folks

[dprotaso]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, Harwayne

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• webhook/OWNERS [dprotaso]

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