Very short duration in examples for Issuing certificates

In many places in the documentation for setting up automatically rotating certificates, a duration of just 24h or 48h is used. That seems very short, considering that `linkerd check` recommends all certificates to be valid for at least 60 days.

I think this should be changed, or at least clarified with a warning that these values should not be used.

I'm not familiar enough with how a proper setup should look like, so I can't help you out with a PR.

## Example

https://github.com/linkerd/website/blob/51748b98e2d5dd7f1f2fa101b11472c8d7439b47/linkerd.io/content/2.11/tasks/automatically-rotating-webhook-tls-credentials.md?plain=1#L115-L116

Start with linkerd.io/content/2.11/tasks/automatically-rotating-webhook-tls-credentials.md, especially the example at lines 115–116, then search the documentation for other 24h and 48h certificate durations. Done means the affected examples consistently use appropriate guidance or clearly warn against unsafe short durations.

Written by the indexing model from the issue text.