docs/deprecated: update status for graphdriver-plugins

[thaJeztah]

• relates to https://github.com/docker/cli/pull/5172
• relates to https://github.com/moby/moby/pull/48050

Add a mention of the feature being disabled by default, and the DOCKERD_DEPRECATED_GRAPHDRIVER_PLUGINS env-var.

- What I did

- How I did it

- How to verify it

- Description for the changelog

Document DOCKERD_DEPRECATED_GRAPHDRIVER_PLUGINS env-var

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

@krissetto @vvoland I tried to rephrase it a bit; let me know if it looks better 🙈