Resolve "Use only one version of the key icon"

What does this MR do?

Simplifies the UI by using only one version of the key icon. Currently we have both key and key-modern, but since they are both representing the same concept of “keys”, one icon will suffice.

As a benefit, this will help decrease the size of the icon library and make it easier to update a single key icon in the future (See gitlab-design#602 (closed)).

Closes #31438 (closed)

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team