Update platform support

[sudoforge]

git-bug has a history of releasing artifacts for many different
platforms and architectures without any explicit support or testing (e.g.
linux/386 and linux/s390x). Additionally, modern architectures are missing
from some platforms (e.g. linux/arm64, */riscv, */riscv64). In an effort
to make more clear the platforms and architectures that are supported, we should
update the platforms and architectures that we generate artifacts for those that
we can actually perform viable testing on, either with physical or virtualized
hardware.

To that end, I propose that future releases provide prebuilt binaries for:

• darwin/amd64
• darwin/arm64
• linux/amd64
• linux/arm64
• linux/riscv64
• windows/amd64
• windows/arm64

... and that users on other platforms are encouraged to build from source (or
otherwise use or provide a package in their package registry of choice).
Stakeholders of platforms not listed above are welcome to open an issue
requesting that we build, test, and upload an artifact for their platform, which
will be triaged independently.