chore(deps): bump @slack/web-api from 6.7.2 to 7.8.0 in /functions

Bumps [@slack/web-api](https://github.com/slackapi/node-slack-sdk) from 6.7.2 to 7.8.0.
- [Release notes](https://github.com/slackapi/node-slack-sdk/releases)
- [Commits](https://github.com/slackapi/node-slack-sdk/compare/@slack/web-api@6.7.2...@slack/web-api@7.8.0)

---
updated-dependencies:
- dependency-name: "@slack/web-api"
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>