v2.1.x Chore - Update license URLs to direct raw content links

[reactive-firewall]

Trivial chore to refactor all relevant file heading's URLs to the new preferred: https://github.com/reactive-firewall/multicast/tree/HEAD/LICENSE.md
or the canonical:
https://raw.githubusercontent.com/reactive-firewall/multicast/HEAD/LICENSE.md