Enable autoFocus of input fields for registration, login, and password reset.

[Matthew-Grayson]

🗣 Description

Add auto focus to first field of components related to sign in, registration, and password reset.
Update placeholder text to improve site accessibility when using a screen reader.
Delete AuthContextOld.tsx, commented out code, and unused imports.

💭 Motivation and context

Closes issue #2260

🧪 Testing

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced
  in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Create a release.

[Matthew-Grayson]

Once PR #2364 is merged, I will resolve any conflicts and mark this ready for review.

[Matthew-Grayson]

I'm testing these changes using a screen reader to rule out potential accessibility issues.

[nickviola]

👍 LGTM

[ameliav]

LGTM!