Bump up GO version to 1.21 (#71)

go.mod

@@ -1,6 +1,6 @@
 module github.com/omec-project/pfcpsim
 
-go 1.16
+go 1.21
 
 require (
 	github.com/c-robinson/iplib v1.0.3
@@ -13,3 +13,13 @@ require (
 	google.golang.org/grpc v1.44.0
 	google.golang.org/protobuf v1.27.1
 )
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	golang.org/x/net v0.0.0-20200822124328-c89045814202 // indirect
+	golang.org/x/sys v0.0.0-20210320140829-1e4c9ba3b0c4 // indirect
+	golang.org/x/text v0.3.0 // indirect
+	google.golang.org/genproto v0.0.0-20200526211855-cb27e3aa2013 // indirect
+	gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c // indirect
+)