[release-0.24] [PingSource] disable @every

[knative-prow-robot]

This is an automated cherry-pick of #5585

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

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[lionelvillard]

@googlebot I consent

[codecov]

Codecov Report

Merging #5590 (04512e0) into release-0.24 (a574b7b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@               Coverage Diff                @@
##           release-0.24    #5590      +/-   ##
================================================
+ Coverage         82.62%   82.65%   +0.02%     
================================================
  Files               198      198              
  Lines              6154     6162       +8     
================================================
+ Hits               5085     5093       +8     
  Misses              742      742              
  Partials            327      327              

Impacted Files	Coverage Δ
pkg/apis/sources/v1/ping_validation.go	100.00% <100.00%> (ø)
pkg/apis/sources/v1beta2/ping_validation.go	100.00% <100.00%> (ø)

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[lionelvillard]

/approve
/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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