[Snyk] Security upgrade @slack/client from 3.9.0 to 5.0.0

[snyk-bot]

Snyk has created this PR to fix one or more vulnerable packages in the `yarn` dependencies of this project.

Changes included in this PR

• Changes to the following files to upgrade the vulnerable dependencies to a fixed version:
  • package.json
  • yarn.lock

Vulnerabilities that will be fixed

With an upgrade:

Severity	Priority Score (*)	Issue	Breaking Change	Exploit Maturity
	551/1000 Why? Recently disclosed, Has a fix available, CVSS 5.3	Regular Expression Denial of Service (ReDoS) SNYK-JS-WS-1296835	Yes	No Known Exploit

(*) Note that the real score may have changed since the PR was raised.

Check the changes in this PR to ensure they won't cause issues with your project.

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