Removing the Slack CLI deno command

We previously mentioned that we had deprecated the deno command and removed its listing from the help command. We have now removed the deno command completely. Developers who were depending on this command should now use the deno executable directly.

We've also removed support for the deprecated, pre-release Deno Slack SDK versions that used slack.yaml and project.ts files. The Run-on-Slack platform no longer supports the slack.yaml file format, and no production projects should be affected. The project.ts file was deprecated by the Deno Slack SDK in favor of manifest.ts and manifest.json files.