Bump jsonwebtoken and @slack/oauth in /slack-service (#37)

Bumps [jsonwebtoken](https://github.com/auth0/node-jsonwebtoken) and [@slack/oauth](https://github.com/slackapi/node-slack-sdk). These dependencies needed to be updated together.

Updates `jsonwebtoken` from 8.5.1 to 9.0.0
- [Release notes](https://github.com/auth0/node-jsonwebtoken/releases)
- [Changelog](https://github.com/auth0/node-jsonwebtoken/blob/master/CHANGELOG.md)
- [Commits](auth0/node-jsonwebtoken@v8.5.1...v9.0.0)

Updates `@slack/oauth` from 2.5.4 to 2.6.0
- [Release notes](https://github.com/slackapi/node-slack-sdk/releases)
- [Commits](https://github.com/slackapi/node-slack-sdk/compare/@slack/oauth@2.5.4...@slack/oauth@2.6.0)

---
updated-dependencies:
- dependency-name: jsonwebtoken
  dependency-type: indirect
- dependency-name: "@slack/oauth"
  dependency-type: indirect
...

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>