fix(cert-manager): Ensure there is at least one leaf certificate renewal when renewing the CA

[tete17]

The renewBefore value for the root ca was simply too low barely giving the leaf certificate any time to renew itself. This leads to the root ca expiring before the leaf certificates expires.

By removing the renewBefore values we go back to the 2/3 default and as long as the leaf certificate is only valid for half of the root it should be fine.

Provide a description of what has been changed

Checklist

• [x] I have verified that my change is according to the deprecations & breaking changes policy
• [x] Commits are signed with Developer Certificate of Origin (DCO - learn more)
• [x] README is updated with new configuration values (if applicable) learn more
• [x] A PR is opened to update KEDA core (repo) (if applicable, ie. when deployment manifests are modified)

Fixes #710