Update member info on serf EventMemberUpdate

[marvinchin]

Description

Fix #27427

Testing & Reproduction steps

Repeated the reproduction in #27427 and verified that it now works.

Happy to add some automated tests if maintainers feel that it's necessary, please feel free to point me to where to add the tests if so.

Reviewer Checklist

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Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

[hashicorp-cla-app]

All committers have signed the CLA.

[tgross]

Hi @marvinchin! Thanks for the PR! A few things to address:

• When the server receives an event on this channel, we update the peersCache with the parsed parts. It looks like if we start accepting EventMemberUpdate that we end up with duplicate entries. So we'll need to de-duplicate those as well.
• Testing is a little gross because we can't easily force the running agent to update and the TestServer cleans itself up on shutdown. We could probably have a test in nomad/serf_test.go that injects the event directly and that'll exercise things like peersCache, but those changes won't show up in the memberlist. That's probably fine though, as that's what we care about here.
• Can I get you to sign the CLA and work up a changelog via make changelog?