This change removes references to Observatory, including:
- Deprecated flags
- Deprecated embedder APIs
- Outdated documentation
- Documentation instances where "VM service" should have been used
- Incorrectly named tests / directories
As a part of this change, `--serve-observatory` is no longer a valid
flag.
Observatory is still available for now via the `_serveObservatory` RPC,
but will be removed in a follow up PR once we've prepared for breakages
in G3.
Work towards https://github.com/dart-lang/sdk/issues/50233
FYI @a-siva
