chore(bigtable): switch to google.golang.org/protobuf

[neild]

Update references to github.com/golang/protobuf to use the newer module. No user-visible changes.

Regenerate bigtable/internal/conformance/tests.pb.go.

[codyoss]

@telpirion Would you mind taking a look at this one.

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[trakhimenok]

Why is this not merging? Would be good to get rid of deps on an obsolete module.

[codyoss]

@trakhimenok I think this issue just fell through the cracks. I think at this point we should just make a new PR. Ill get a new one out today and get this reviewed.