Migrate Pajamas button in _signup_omniauth_provider_list.haml

What does this MR do and why?

• #417815 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Feature flag (new design)	Feature flag disabled (old one)
migration-button-omniauth-ff-on	migration-button-omniauth-ff-off

How to set up and validate locally

• If you disabled the sign-up, go the Admin > Settings > General > Sign-up restrictions and enable sign-up first.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.