dev: drop outdated build scripts + minor updates (sourcegraph#53841)

This PR drops all now outdated `build.sh`, `Dockerfile*` from artifacts
entirely built from Bazel. It also updates `sg.config.yaml` to replace
`install` steps that were using them.

## Test plan

<!-- All pull requests REQUIRE a test plan:
https://docs.sourcegraph.com/dev/background-information/testing_principles
-->

- Ran locally all new sg targets
- CI 
- 🟢 Main-dry-run
https://buildkite.com/sourcegraph/sourcegraph/builds/229439

featureflags: remove cody-gateway-management-ui, enable by default (s…

…ourcegraph#53585)

As we approach GA, this enables the Cody Gateway management UI by
default for Sourcegraph.com so that teammates can begin to familiarize
themselves with it before Enterprise GA (5.1 release). I'll be making an
announcement tomorrow 😁

Closes sourcegraph#52519

### Test plan

CI

Add stop generating button to cody web (sourcegraph#53191)

Add stop generating button for in progress message for cody in web.
<img width="760" alt="image"
src="proxy.php?url=https%3A%2F%2Fgithub.com%2F%3Ca+href%3D"https://github.com/sourcegraph/sourcegraph/assets/22571395/8ea16443-a125-4a73-8859-eb815cdd89e3">https://github.com/sourcegraph/sourcegraph/assets/22571395/8ea16443-a125-4a73-8859-eb815cdd89e3">


https://www.loom.com/share/811352d2fb6942ff9851c9dedda281e1
## Test plan

- visit /cody
- ask a question
- click on stop generating button
- reload the page

Cody: display error on older SG version or when cody is not enabled(s…

…ourcegraph#52739)