Use `unknown` instead of `any` for `StringIndexed`

[robhung]

https://github.com/slackapi/bolt-js/blob/e14c6a19f9435607745daf16aab4062cf6b731a7/src/types/helpers.ts#L5

As per title, I think it should be unknown to prevent accessing properties which may not exist.

[seratch]

Thanks for the suggestion! This can be a breaking change, so we are not planning to make this change happen in the short term. In the long run, changing the type when upgrading the major version may be worth considering.