"Set up an administration password" and "How to use bridges in Tails" doc pages are not updated for the new Greeter

Originally created by @intrigeri on #14790 (Redmine)

https://tails.boum.org/doc/first_steps/startup_options/administration_password/ talks about “Yes” and “Forward” buttons.

Same for “How to use bridges in Tails” on https://tails.boum.org/doc/first_steps/startup_options/bridge_mode/.

Candidates for core work tech writing?

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