Fix ShutdownCommand message styling

[456dev]

the docs specify the command as accepting json components, as well as minimessage
in the previous implementation, neither were true, as the minimessage deserialisation after parsing legacy escaped any tags internally.

This PR fixes that by only deserialising the string once before parsing it on.
due to this, legacy "&" support has been removed

to support parsing as both minimessage + json, parsing as json will only kick in if it looks like a valid json text component:

• "plain format, starting with a quote."
• {"text":"Object format, starting with a curly brace"}
• ["Array format", {"text": "starting with a square bracket"}]

TODO: docs pr to document json format hinting, as well as more strongly say that it only works in velocities console.