Bump proxy minimum version to 0.8.3

[AxelTheGerman]

https://github.com/basecamp/kamal-proxy/releases/tag/v0.8.3

• Populate X-Request-Start header
• Log client disconnections with status 499
• Use an identifiable User-Agent header on healthcheck requests
• When target fails to become healthy, mention timeout in error message

Note: I'm also happy to implement a mechanism that doesn't require bumping the minimum version for every patch release. See #1252

[AxelTheGerman]

@djmb do you think this could get merged or would you prefer having a solution that does not require bumping this version constantly and saving folks who don't care about it a proxy reboot?

[djmb]

I've merged that, but looks like we need to bump it again for 0.8.4 😅

Regarding allowing users to choose the version, we should certainly allow patch version updates.

Kamal proxy hasn't reached 1.0 yet so we may want to make some breaking changes before then. But we don't have a backward or forward compatibility policy yet so it is safest to force the versions to be in sync right now.

[AxelTheGerman]

Kamal proxy hasn't reached 1.0 yet so we may want to make some breaking changes before then. But we don't have a backward or forward compatibility policy yet so it is safest to force the versions to be in sync right now.

I assume at that point it would make sense to have a ruby gem style syntax for the proxy, e.g. ~> 1.2 or '>= 2.2.1', '< 3.0'