chore: remove the Slack package

[bgatellier]

• Remove the package from the repository
• Remove the unreleased changelogs related to the package
• Remote any mention of the package in the documentation

[changeset-bot]

⚠️ No Changeset found

Latest commit: 7c2a0b4

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes changesets to release 8 packages

Name	Type
@scodi/ssllabs-server	Major
@scodi/observatory	Major
@scodi/lighthouse	Major
@scodi/greenit	Major
@scodi/common	Major
@scodi/mysql	Major
@scodi/api	Major
@scodi/cli	Major

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Click here if you're a maintainer who wants to add a changeset to this PR