Audit and update buttons on SessionsController#new

What does this MR do?

• Updates the Sign in button to use the confirm button variant.
• Updates the social login buttons to use the default button variant.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

N/A

Security

N/A

Related to #273292 (closed)