Resolve "Migrate or remove instances of deprecated issues icons in product"

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What does this MR do and why?

This PR removes the deprecated icons from the GitLab EE.

Screenshots

The following screenshots are examples, not an exhaustive list of all changes in this MR.

issues ➡️ issue-type-issue SVG

Currently, the issues svg is identical to the issue-type-issue svg, so there are no visible changes in the UI.

Location	issues svg	issue-type-issue svg
Issue board filtered search
Customer relations contacts
Issues dashboard
Sidebar

epic-closed ➡️ issue-closed

epic-closed svg has been replaced with issue-close svg.

Location	epic-closed svg	issue-close svg
Contributor events

issue-closed ➡️ issue-close

issue-closed svg is to be replaced with issue-close svg.

Before	After

How to set up and validate locally

Search for deprecated icons in the codebase to confirm none remain:

• git grep -l "issue-closed" -- app
• git grep -l "epic-closed" -- app
• git grep -l "issue-update" -- app
• git grep -l "['"]issues['"]" -- app
• git grep -l "['"]requirements['"]" -- app

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #526952