This change removes the remaining usages of Syntax() from Go Protobuf
and uses edition features instead.
It also adds a new function to the EnumDescriptor interface checking if
the enum is using a Closed semantics.
All of these changes were tested on the Google corpus.
Change-Id: I7a8110f6f3b6ed24bf7ece500b4942371302c56c
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/572695
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>