Add an explicit feature flag for IS-IS multi-topology support

[jbemmel]

And don't require it for ipv6-only scenarios

SR Linux removed support for IS-IS multi-topology in 23.10, but can still support ipv6-only deployments

Fixes #1169

[ipspace]

No.

You're adding a feature flag and modifying 5 device definitions (missing at least two others) just because you have a problem with a single device. Solve it with a device quirk.

Also, saying "we'll use multi-topology only in dual-stack deployments" doesn't help. What if part of the network is dual-stack and part of it is IPv6-only?

Finally, you fixed FRR template (because you care about that), but not all the others that also configure multi-topology. That's not how things work.

If you care about running IPv6 with IS-IS on SR Linux, please create a plugin that would implement all the IS-IS deployment scenarios you'd like to see, and add configuration templates for whichever devices you think should be supported by that plugin.