Resolve "Add size 96 to avatar sizes in avatar.scss"

What does this MR do?

Adding 96 size to the avatar sizes will allow us to use the 96 size which adheres to Avatars in Pajamas, as 96 is the max size shown in the Avatar Design Specs.

Adding the 96 value will add .s96 class that could be used across gitlab-ce and gitlab-ee when we need to use an avatar with the size of 96.

Currently available classes close to that size are: 90 and 100.

This change is blocking gitlab-ee!14660

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

Performance and testing

• 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

Closes #64499 (closed)