Tag deprecated attributes with the CompletionItemTag

[rcjsuen]

docker-language-server/internal/compose/compose-spec.json

Lines 9 to 13 in 9f2f18e

	"version": {
	"type": "string",
	"deprecated": true,
	"description": "declared for backward compatibility, ignored. Please remove it."
	},

Some attributes in the Compose schema have been deprecated. We should tag such completion items with the Deprecated tag so that the client can render this to the user such as for the MAINTAINER keyword in a Dockerfile.