Decrease timeout of cert-manager webhook

[robertgraeff]

What this PR does / why we need it:

The cert-manager chart sets a default timeout of 30 seconds in its validating webhook configuration. Gardener however allows only a maximum of 15 seconds. This pull request sets the cert-manager's helm chart value to fulfill the Gardener constraint.

Details

The MCP clusters in the Gardener dashboard are marked with a red exclamation mark indicating that the clusters have an issue: "...ValidatingWebhookConfiguration "cert-manager-webhook" is problematic...". The ValidatingWebhookConfigurations have the annotation:

metadata:
  annotations:
    gardener.cloud/warning: |-
      ATTENTION: This webhook configuration has been modified by Gardener since it does not follow the 
      best practices recommended by Kubernetes 
      (https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/#best-practices-and-warnings) 
      and might interfere with the cluster operations. Please make sure to follow these recommendations to 
      prevent future interventions. When you are done, please remove this annotation. See also 
      https://github.com/gardener/gardener/blob/master/docs/usage/shoot/shoot_status.md#constraints 
      for further information.
      The following modifications have been made:
      - timeoutSeconds of webhook "webhook.cert-manager.io" was set to 15

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

- Decrease timeout of cert-manager webhook.