chore: migrate from using `webhook.Defaulter` and `webhook.Validator`

[SaphMB]

controller-runtime's webhook.Defaulter and webhook.Validator have been deprecated in favour of webhook.CustomDefaulter/CustomValidator since v0.17 and will be removed with v0.20.

We are currently using v0.18, putting us in a good position to pre-empt the breaking change, upgrade to v0.19 and transition to v.20 without this issue posing a problem.

Notes

• v0.19 release notes
• Upcoming changes to kubebuilder