Add bash v4 to requirements

[julz]

The requirement of some of our dev scripts on bash v4+ confused at least one person in #9792, so seems worth documenting.

/assign @n3wscott @mattmoor

[codecov]

Codecov Report

Merging #9797 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9797      +/-   ##
==========================================
+ Coverage   87.72%   87.77%   +0.04%     
==========================================
  Files         180      180              
  Lines        8553     8553              
==========================================
+ Hits         7503     7507       +4     
+ Misses        799      796       -3     
+ Partials      251      250       -1     

Impacted Files	Coverage Δ
pkg/reconciler/configuration/configuration.go	89.16% <0.00%> (+1.66%)	⬆️
pkg/autoscaler/scaling/multiscaler.go	89.74% <0.00%> (+1.70%)	⬆️

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[markusthoemmes]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: julz, markusthoemmes

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[dprotaso]

there's actually a debate about this going on here:
knative/test-infra#2143