`BgpPeer::communities` - use a set instead of a vec?

[jgallagher]

BgpPeer::communities is currently a Vec<u32>:

omicron/nexus/types/versions/src/bgp_unnumbered_peers/networking.rs

Lines 163 to 164 in 6a3bdd6

	/// Include the provided communities in updates sent to the peer.
	pub communities: Vec<u32>,

However, I think it would be more accurate to make this a BTreeSet<u32>:

• Inserting and then reading back from the db does not preserve order
• We don't expect duplicate values - if the Vec has duplicates in it, insertion will fail with an ObjectAlreadyExists error

(Like #10121, this came up while extending tests in support of the #9832 work.)