refactor(Avatar): Refactor Avatar component to use CSS modules behind feature flag

[jonrohan]

Part of https://github.com/github/primer/issues/3877

Changelog

Changed

Converting Avatar component to use CSS modules behind the primer_react_css_modules_team feature flag

Rollout strategy

• [ ] Patch release
• [x] Minor release
• [ ] Major release; if selected, include a written rollout or migration plan
• [ ] None; if selected, include a brief description as to why

Testing & Reviewing

Tests updated for in and outside of feature flag.

Merge checklist

• [ ] Added/updated tests
• [ ] Added/updated documentation
• [ ] Added/updated previews (Storybook)
• [ ] Changes are SSR compatible
• [ ] Tested in Chrome
• [ ] Tested in Firefox
• [ ] Tested in Safari
• [ ] Tested in Edge
• [ ] (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)