Update minimum k8s version to 1.25

Now that we support CEL in our own custom CRDs, we should update our minimum supported version of k8s to be 1.25.

**Acceptance**
- Update versions doc for k8s min version
    - sidenote: also need to update current nginx version to 1.25.5
- Remove references to k8s 1.23/1.24 and webhook installation
- automated tests use 1.25 as minimum version
