Use `require` rather than `replace` for dtls version.

This is an update to !66 (merged). I think that replace is needed only when rewriting the URL of a dependency. To update the version number only requires go get.

go mod edit -dropreplace=github.com/pion/dtls/v2
go get github.com/pion/dtls/v2@v2.0.12

go.mod

@@ -6,6 +6,7 @@ require (
 	git.torproject.org/pluggable-transports/goptlib.git v1.1.0
 	github.com/google/uuid v1.2.0 // indirect
 	github.com/gorilla/websocket v1.4.1
+	github.com/pion/dtls/v2 v2.0.12 // indirect
 	github.com/pion/ice/v2 v2.0.15
 	github.com/pion/sdp/v3 v3.0.4
 	github.com/pion/stun v0.3.5
@@ -21,5 +22,3 @@ require (
 	golang.org/x/net v0.0.0-20211201190559-0a0e4e1bb54c
 	google.golang.org/protobuf v1.23.0
 )
-
-replace github.com/pion/dtls/v2 => github.com/pion/dtls/v2 v2.0.12