[release-0.23] [PingSource] disable @every

[knative-prow-robot]

This is an automated cherry-pick of #5585

:bug: PingSource schedules starting with `@every` are now properly rejected.

[google-cla]

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-0.23@a5a8f89). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff               @@
##             release-0.23    #5591   +/-   ##
===============================================
  Coverage                ?   82.74%           
===============================================
  Files                   ?      194           
  Lines                   ?     6015           
  Branches                ?        0           
===============================================
  Hits                    ?     4977           
  Misses                  ?      716           
  Partials                ?      322           

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[lionelvillard]

@googlebot I consent
/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[lionelvillard]

@googlebot I consent