Major: Slack archived the JSON Schema repo

As the title says; the official @slackapi JSON Schema is officially dead: https://github.com/slackapi/slack-api-specs

It's a shame. It was never really up to date, and it's never gonna be. That's why we have to build this huge patch for missing parameters, methods, etc: https://github.com/jolicode/slack-php-api/blob/main/resources/slack-openapi-sorted.patch

I think that on the long run this Slack PHP API Client should not rely on the deprecated spec anymore.

We need to find another way. All we need is a up to date OpenAPI Schema. Maybe we should build it.

- Scrape https://docs.slack.dev/
- Extract from https://github.com/slackapi/python-slack-sdk which is the most up to date client
- Any other idea?

Also, someone discovered a JSON Schema for Block Kit hidden in some tools: https://gist.github.com/renatorib/1fb1a9bd71435b41bee602d15bc56899

Let's also ping some dev advocate here: @mwbrooks @wongjas