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

requested review from @meskio

Adding @meskio as reviewer since at the meeting today we discussed updating docker packages etc.

On second thought—I'm not sure this works. pion's own dependencies (e.g. pion/webrtc) will still have the old version number for pion/dtls set. Let me do a test and I'll write back.

added 30m of time spent

resolved all threads

Sounds good, I'll merge it.

merged