`ClusterHealthRequestTests.testBwcSerialization` shouldn't be a unit test

[arteam]

There is a specific set of tests which deals with BWC according to the backward compatibility guarantees of ES as a product. testBwcSerialization manually checks the compatibility all the way back to 7.2.0.

[DaveCTurner]

I'm relabelling this as >test-failure since it represents a muted test that needs further action, and adding an area label. That way it'll be tracked in the team's test failures report. (I see why >test-mute looked like the right choice in this case but actually this label is just for PRs that mute tests).

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)