Remove useOctavia setting

[lotharbach]

How to categorize this issue?

/area networking
/kind enhancement
/platform openstack

What would you like to be changed:
kubernetes/cloud-provider-openstack#1882 removes cloudprovider.conf flag use-octavia from occm v1.26.0 and newer. This is all useOctavia ever was used for anyway, so this is already a noop setting for anything but Shoots running k8s v1.25 or older.

Why is this needed:
I'm aware k8s v1.25 is still supported by gardener and this extension at the moment, but I don't think users of this setting are aware that it is suddenly going to be ignored when they go to update a shoot, and removing it in a breaking change would be a much clearer call to action than any alternative that I can think of. In reality I expect nobody is using LBaaS v1 anyway.