Bump @slack/types version to ^2.8.0

[zimeg]

Summary

This PR bumps the version of @slack/types to ^2.8.0 – the current version.

Notes

This bump was done to fix the following build error, however I now cannot replicate this with @slack/types@2.7.0 so I am unsure of what actually caused this... However, this upgrade seems appropriate regardless 😃

src/App.ts:1554:11 - error TS2322: Type 'RespondArguments | { text: string; }' is not assignable to type 'RespondArguments'.
  Type '{ text: string; }' is not assignable to type 'RespondArguments'.
    Property 'metadata' is missing in type '{ text: string; }' but required in type 'Pick<ChatPostMessageArguments, "metadata" | "mrkdwn" | "token" | "thread_ts" | "as_user" | "attachments" | "blocks" | "link_names
" | "parse" | "icon_emoji" | ... 4 more ... | "unfurl_media">'.

1554     const normalizedArgs: RespondArguments = typeof message === 'string' ? { text: message } : message;
               ~~~~~~~~~~~~~~


Found 1 error in src/App.ts:1554

Requirements

• 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 #1838 (0dfcc17) into main (ba5e2b8) will not change coverage.
The diff coverage is n/a.

❗ Current head 0dfcc17 differs from pull request most recent head 4b58356. Consider uploading reports for the commit 4b58356 to get more accurate results

@@           Coverage Diff           @@
##             main    #1838   +/-   ##
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  Coverage   82.15%   82.15%           
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  Files          18       18           
  Lines        1519     1519           
  Branches      435      435           
=======================================
  Hits         1248     1248           
  Misses        175      175           
  Partials       96       96           

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