bigtable: Update user-agent to the latest version (#3429)

Currently the user agent of Bigtable in Go is standalone, as in https://github.com/googleapis/google-cloud-go/blob/master/bigtable/doc.go#L121m. Update it to the latest version https://pkg.go.dev/cloud.google.com/go/bigtable. Related issue: #3330.

Background: DirectPath will use user agent to deny a specific library version, so we want to update the user agent with the library release date.

bigtable/doc.go

@@ -117,8 +117,8 @@ const (
 )
 
 // clientUserAgent identifies the version of this package.
-// It should be bumped upon significant changes only.
-const clientUserAgent = "cbt-go/20180601"
+// It should be the same as https://pkg.go.dev/cloud.google.com/go/bigtable.
+const clientUserAgent = "cbt-go/v1.6.0"
 
 // resourcePrefixHeader is the name of the metadata header used to indicate
 // the resource being operated on.