update goreleaser configuration

[slingamn]

1. Right now we build freebsd-386 and windows-386; stop building these
2. We build all three of linux-armv6, linux-armv7, and linux-arm64; we should probably remove linux-armv7 (affected systems should be able to run the v6 binary?)
3. We do not build macos-arm64, but we should
4. We should build plan9-amd64 because lol
5. We should probably build openbsd-amd64?
6. Include LICENSE in the release
7. We should start including a GPG signature of the checksums file (this is a process thing that has to be documented in DEVELOPING.md, not a configuration issue)