Allow cert rotation to store less backup tls directories

[rancher-max]

By default, I believe this value is unlimited. We should allow users to be able to specify how many backups they may want, and by default only store 2-5. In current state, you could have a directory like the following after doing multiple cert rotations, which may be undesired on some systems.

# /var/lib/rancher/k3s/server
tls
tls-81281295
tls-81281300
tls-81281305
tls-81281310
tls-81281315
tls-81281320
tls-81281325
tls-81281330
tls-81281335

I'd expect it to work similar to the following, where the number specified is how many backup files to store:

$ k3s server --cert-rotation-retention=5

...
# /var/lib/rancher/k3s/server
tls
tls-81281315
tls-81281320
tls-81281325
tls-81281330
tls-81281335

[caroline-suse-rancher]

Closing, as we now offer cert rotation