Add Linear logo for integration

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Purpose

Downloaded from https://linear.app/brand

When referring to Linear as a company, such as on social media, or where a “chip” design is required, it is acceptable to use this stylized icon with an appropriate corner radius.

See: gitlab!198297 (merged)

Screenshots

Checklist

For a new icon and/or illustration:

• Link to related design issue:
• If this adds an icon ensure that it follows the new icon checklists.
• If this adds an illustration ensure that it follows the new illustration checklists.

For third-party trademarks:

• If this adds or updates an icon or illustration that includes a third-party trademark, ensure that it follows the process for adding third-party trademarks to GitLab.