gitea-backend: fix config casing

[oliverpool]

Summary

Currently the gitea AuthenticationPage refers to {config.logoUrl} whereas it should be {config.logo_url} like the other backends

Test plan

Set a custom logo_url and the backend to gitea. Your custom logo should appear on the login page.

Checklist

Please add a x inside each checkbox:

• [x] I have read the contribution guidelines.

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