Fix milestone icon composition

Purpose

Adjusts shape sizing and stroke composition, per !1043 (comment 1827507950)

Screenshots

Checklist

For a new icon and/or illustration:

• Link to related design issue: #404 (closed)
• 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.