Adding new work item type icons to replace existing issue type icons

Purpose

In #438 (moved) we have introduced a new work-items icon, while also updating the terminology of the other associated type icons to use the work-item name rather than issue-type. This MR is to add the new icons based on the new naming, so we can migrate any existing icons in the product to these new ones before deprecating them.

legacy	replacement
issue-type-enhancement	work-item-enhancement
issue-type-feature-flag	work-item-feature-flag
issue-type-feature	work-item-feature
issue-type-incident	work-item-incident
issue-type-issue	work-item-issue
issue-type-keyresult	work-item-keyresult
issue-type-maintenance	work-item-maintenance
issue-type-objective	work-item-objective
issue-type-requirements	work-item-requirement
issue-type-task	work-item-task
issue-type-test-case	work-item-test-case
issue-type-ticket	work-item-ticket

This similarly creates the following icons to replace legacy icons based on the updating naming:

legacy	replacement
issue-new	work-item-new
epic	work-item-epic

Screenshots

Note

To reiterate – these icons are being added to replace others as shown, but no icons are being removed yet

issue-type	work-item

issue-new	work-item-new

epic	work-item-epic

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 #438 (moved)