Removed reference to Solokeys and adjusted the language slightly

Why is this change being made?

There are limitations with Solokeys as opposed to YubiKeys (one cannot register two Solokeys with GitLab) and a slowness to adapt, plus we've worked out a way to order YubiKeys at a group rate with no expense reports needed, so only YubiKeys should be mentioned in the handbook for team members. Currently YubiKeys and Solokeys are both mentioned.

There are also references made to "either key" but other Solokey references have already been deleted by someone else, making for oddly-worded paragraphs, so this also cleans that up as well.

Author Checklist

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