infra: remove stale dockerfiles from repo (#28243)

deleting the following docker things from monorepo. they aren't
currently usable because of old dependencies, and I'd rather avoid
people using them / having to maintain them

- /docker
- this folder has a compose file that spins up postgres,pgvector
(separate from postgres and very stale version),mongo instance with
default user/password that we've gotten security pings about before. not
worth having
- also spins up a custom dockerfile with onttotext/graphdb - not even
sure what that is
- /libs/langchain/dockerfile + dev.dockerfile
  - super old poetry version, doesn't implement the right thing anymore
- .github/workflows/_release_docker.yml, langchain_release_docker.yml
  - not used anymore, not worth having an alternate release path