mark multicluster.yaml as required

[michi-covalent]

i removed multicluster.yaml from the required status check list since it's been failing a lot.

multicluster.yaml https://github.com/cilium/cilium-cli/actions/workflows/multicluster.yaml?query=branch%3Amain+event%3Aschedule++
from                to                  duration success rate
2024-05-15 21:04:04 2024-05-16 15:03:42 22m27s   🙀 25% 1/4
2024-05-14 21:03:54 2024-05-16 15:03:42 22m26s   🙀 25% 2/8
2024-05-12 21:03:41 2024-05-16 15:03:42 24m6s    🙀 38% 6/16
2024-05-08 21:03:51 2024-05-16 15:03:42 24m25s   🙀 41% 13/32
2024-04-30 21:04:32 2024-05-16 15:03:42 24m45s   🙀 42% 27/64

i chatted a bit with @giorio94 offline about how to mitigate the issue. here are some options:

• disable flow validation.
• remove multicluster.yaml altogether (assuming https://github.com/cilium/cilium/blob/main/.github/workflows/conformance-clustermesh.yaml gives us enough coverage).
• leave it as not-required until either we find somebody to look into the actual issue or https://github.com/cilium/design-cfps/pull/9 gets implemented and cilium-cli gets merged into cilium repo.

i'll leave it up to @cilium/sig-clustermesh to decide how to handle this 🚀🙏