dev/ci: switch 100% to stateless agents

Created by: bobheadxi

Part of https://github.com/sourcegraph/sourcegraph/issues/33184 , closes https://github.com/sourcegraph/sourcegraph/issues/30753

Test plan

This has been in partial rollout for a while. Things have been stable and healthy it seems, with the only issues being those shared across both stateful and stateless agents at the moment

• metrics
• step failures

main-dry-run: https://buildkite.com/sourcegraph/sourcegraph/builds/139658