Bump the oldest supported version of node-slack-sdk to support modals for sure (cherry-picking #287)

[seratch]

Summary

With @slack/web-api 5.0.0, Block Kit in modals doesn't work in Bot apps as necessary ones are not available in such old versions. #306 (comment) is an issue caused by that.

Block Kit in modals are available in @slack/web-api 5.2.1.

• https://github.com/slackapi/node-slack-sdk/releases/tag/%40slack%2Fweb-api%405.2.0
• https://github.com/slackapi/node-slack-sdk/releases/tag/%40slack%2Fweb-api%405.2.1

This pull request cherry-picked only the change resolving it from @PerStirpes 's PR #287 .

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #322 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #322   +/-   ##
=======================================
  Coverage   73.71%   73.71%           
=======================================
  Files           7        7           
  Lines         506      506           
  Branches      145      145           
=======================================
  Hits          373      373           
  Misses        101      101           
  Partials       32       32

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[stevengill]

LGTM.

New functionality being added in a minor release is totally acceptable. It wouldn't warrant a major version bump as nothing broke.

It would have been nice if we updated the dependencies alongside adding the functionality. Good to keep in mind next time. Wonder if we can tweak our testing env to catch something like this.