Remove obsolete Docker version startup check

[khalilswdp]

Removes the startup check that asserts the Docker server version is at least 1.6.0.
Docker 1.6.0 was released in 2015, and every daemon the modern client libraries can
connect to is far past it, so the check can never fail in practice — it only adds a
version parse and log output to every first container start.

Changes

• Remove checkDockerVersion and the now-unused private check helper from
  DockerClientFactory, along with the surrounding checks block in client().
  Both methods were private, so there is no public API impact.
• Remove the corresponding line from the startup-checks example in
  docs/features/configuration.md.

Notes

• The checks.disable configuration property is intentionally left in place: it is
  still consumed elsewhere (ContainerDef), and whether to deprecate it is left as
  a follow-up decision.
• The remaining lines in the docs startup-checks example ("File should be mountable",
  "A port exposed by a docker container should be accessible") describe checks that
  were removed some time ago; happy to rewrite that docs section here or in a
  follow-up, whichever is preferred.