Update Slack logo in GitLab SVG to not include empty space
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=350437)
</details>
<!--IssueSummary end-->
The following discussion from !78094 should be addressed:
- [ ] @ekigbo started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/78094#note_810503539): (+2 comments)
> **question:** could we use a bigger svg? This feels like it could break if we update the logo :thinking:
issue