ci: Pull updates from Caddy, build .deb as well

[francislavoie]

FYI @mohammed90 @mholt

Three commits:

1. brings the release.yml in line with the caddy repo's, and adds simple .deb packaging (no extra files or scripting, just the /usr/bin/xcaddy)
2. brings the ci.yml in line with the caddy repo's, including testing on Go 1.15
3. adds Cloudsmith .deb publishing (which is why I'm marking this as draft, until ci: Add pushing to cloudsmith caddy#3941 is complete and tested)

[mholt]

Thanks!

Should I tag a new release here in xcaddy first and see if it works? Before tagging the 2.3 release of caddy.

[francislavoie]

Yeah we could do that too.