config: mention that maxfiles is deprecated

config: mention that maxfiles is deprecated

Don't add an explicit deprecation warning on parsing (yet), this already done in
the pve6to7 script. Also, automatic conversion to 'prune-backups' happens when
the section config is read, so over time fewer users should be affected.
Postpone explicit warning/dropping the parameter to a future major release.

Also switch the setting for the default 'local' storage to 'prune-backups'.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>