Updated GRPC Server Authentication Test Documentation to use Meta.Key

[tmulle]

Fix #43811
Redid the doc changes in a single commit

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It should be safe to merge provided you have a look at the other checks in the summary.

[gsmet]

Makes sense to me, thanks, but I will let @alesj confirm!

[alesj]

@gsmet lgtm
@tmulle when did the change from String to Metadata.Key got introduced?

[tmulle]

I'm not sure the exact change, but the latest version 1.67.1 of io.grpc.Metadata shows the MetaData.Key
and the version in my project is 1.65.1 and that also uses the MetaData.Key

https://grpc.github.io/grpc-java/javadoc/io/grpc/Metadata.html