Release: Slack CLI v2.21.0

Version 2.21.0 of the developer tools for the Slack automations platform has sprung!

• You can now display runtime versions for your application's execution environment using the doctor hook. To enjoy the improved slack doctor command experience, existing Deno Slack SDK-based apps must have their deno-slack-hooks dependency upgraded to the latest version (v1.3.0).
• By relying only on the get-manifest hook when attempting to get a manifest rather than checking for a slack.yaml file, we now surface any errors that are returned when getting the manifest.
• We removed the possibility of an error when generating the message boundary hook protocol, and now preserve spacing in logged outputs.
• We corrected a few typos behind the scenes.