Update @slack/web-api dependency

[aoberoi]

Description

The current version specifier, ^5.2.1, is technically correct. However, there are a number of features added since then that would be useful to most developers.

If you installed Bolt a while ago and want to update, npm update won't help because it only looks for new versions of direct dependencies (not transitive dependencies like @slack/web-api). The alternative is to do rm -rf node_modules && rm package-lock.json && npm install. It's not the worst thing in the world, but its also not worth getting confused over when a feature is missing. The only cost I can see is that if the app takes a direct dependency on @slack/web-api (a minority use case), then and the version the app has chosen doesn't resolve for the version in Bolt (a super minority case), then you end up with two versions in the dependency tree - which just means the npm install operation takes marginally more time and with marginally more disk space.

Let's update it to the latest, which at this point is ^5.6.0.

Requirements (place an x in each of the [ ])

• 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.
• I've searched for any related issues and avoided creating a duplicate issue.

[stevengill]

Fixed in #335