Use confluence icon for Confluence nav item

What does this MR do?

This MR adds a new Confluence icon to use in our project side nav for the Confluence integration.

For why this icon has been added to app/assets/images/, see gitlab-svgs!596 (comment 378557302) from @tauriedavis.

• Issue: #220934 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content