Restore cog_runtime support with pinned coglet version

[michaeldwan]

Turns out I can't remove cog-runtime support entirely. We're still moving towards porting cog-runtime back into cog as the default, but that's not ready yet. So what we'll do until then is:

1. honor the cog_runtime setting in cog.yaml so models that have opted in can continue to use it
2. pin coglet (the python model library in cog-runtime) to a version that predates the concurrency and isolation work in cog-runtime (the go server/runner portion.) That stuff is unfortunately necessary to complete, but we're not comfy exposing existing models to it until we port coglet back to cog and its large test suite. This applies to both cog_runtime: true and fast: true models!
3. stop showing "major upgrade" warnings or opt-in prompts to ensure we don't encourage more usage until we're confident everything works and have integration tests verifying it keeps working

[tempusfrangit]

We will also want to ensure we are adhering to the right version max in monobase.

[michaeldwan]

@tempusfrangit good catch! I think I got it set in the only place that coglet is referenced. Based on this #2349 it doesn't look like anything is using R8_COGLET_RUNTIME, and based on this https://github.com/replicate/monobase/blob/5f8525f38ca916cfef0f55eb9270ab874a1c388b/README.md?plain=1#L88 it seems like setting R8_COG_RUNTIME to a url should work correctly.