Update dependency @slack/web-api to v7.8.0 (#3266)

This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [@slack/web-api](https://slack.dev/node-slack-sdk/web-api) ([source](https://redirect.github.com/slackapi/node-slack-sdk)) | [`7.7.0` -> `7.8.0`](https://renovatebot.com/diffs/npm/@slack%2fweb-api/7.7.0/7.8.0) | [![age](https://developer.mend.io/api/mc/badges/age/npm/@slack%2fweb-api/7.8.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://developer.mend.io/api/mc/badges/adoption/npm/@slack%2fweb-api/7.8.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://developer.mend.io/api/mc/badges/compatibility/npm/@slack%2fweb-api/7.7.0/7.8.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://developer.mend.io/api/mc/badges/confidence/npm/@slack%2fweb-api/7.7.0/7.8.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>slackapi/node-slack-sdk (@&#8203;slack/web-api)</summary>

### [`v7.8.0`](https://redirect.github.com/slackapi/node-slack-sdk/compare/@slack/[email protected]...@slack/[email protected])

[Compare Source](https://redirect.github.com/slackapi/node-slack-sdk/compare/@slack/[email protected]...@slack/[email protected])

</details>

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