Resolve "Update issue-type icons to use default icon style"

Purpose

Update issue-type icons to align with default stroke icon style.

Screenshots

(See #390 (closed) for before/after)

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.

Closes #390 (closed)