Is your feature request related to a problem? Please describe.
yes. #3773 introduced garbage collecting certificateRequest resources. But, the default revisionHistoryLimit is nil, which means revisions will be not be garbage collected, by default.

Describe the solution you'd like
default revisionHistoryLimit should be set to 1, if not specified in the Certificate, through CRD defaulting.

Describe alternatives you've considered
N/A

Additional context
completely agree with @wallrj comments here: #3773 (review)

the default user experience of cert-manager shouldn't require users to explicitly set revisionHistoryLimit >=1.

/kind feature