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 <aktau@google.com>
Reviewed-by: Nicolas Hillegeer <aktau@google.com>
diff --git a/integration_test.go b/integration_test.go
index 0cdb656..33ac34d 100644
--- a/integration_test.go
+++ b/integration_test.go
@@ -43,7 +43,7 @@
 		switch runtime.GOOS + "/" + runtime.GOARCH {
 		case "darwin/arm64":
 		default:
-			vers = []string{"1.11.13", "1.12.17", "1.13.15", "1.14.15", "1.15.15"}
+			vers = []string{"1.13.15", "1.14.15", "1.15.15"}
 		}
 		return append(vers, "1.16.15", "1.17.12", "1.18.4")
 	}()