fix: Update supa_socials_auth.dart to include slackOidc and linkedInOidc OAuthProvider value mappings

[Oskar-Nilsen-Roos]

What kind of change does this PR introduce?

fix

What is the current behavior?

Passing OAuthProvider.linkedinOidc or OAuthProvider.slackOidc provides no icon and no color styling.

What is the new behavior?

Provides the same existing styling & icon for linkedinOidc and slackOidc as their non-oidc counterparts.

Additional context

Related to other PR in supabase-flutter to add slackOidc provider option: https://github.com/supabase/supabase-flutter/pull/1066