SSBC goes Beta with self-hosted executors

Created by: eseliger

We name SSBC a beta feature. It will not be behind a feature-flag by default, it will not impair existing customers using the src-cli workflow. We want every customer to be aware of it.

Depending on the outcome of dogfooding from EM3.6, we should decide as a team whether or not to spend an additional sprint polishing and bugfixing or publish what we have as beta.

• Implementing QA/bug fixes 0.5d - 1d
• https://github.com/sourcegraph/sourcegraph/issues/34251 1d @courier-new
• Release prep: Enable feature flag by default 0.5d @eseliger
• #36920 (closed) @courier-new

This will ultimately close #27841 (closed).