integration test: drop Go 1.11 and Go 1.12
integration test: drop Go 1.11 and Go 1.12

fixes https://github.com/golang/protobuf/issues/1483

Change-Id: Ia5235f5923bc2430213d38bf5c6c5d0bcf25fa24
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/430516
Run-TryBot: Nicolas Hillegeer <178184bf77e7c5ae86300dd7107a89212029c9e8@google.com>
Reviewed-by: Nicolas Hillegeer <178184bf77e7c5ae86300dd7107a89212029c9e8@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go