Upgrade factory_girl to factory_bot

Upkeep for our test suite, from :

• Replace all uses of static attributes in factories with dynamic attributes, required for v4.11.0 onwards.

• Install the gem factory_bot instead of its now-deprecated predecessor factory_girl.

• Update the pickle gem, since otherwise creating users in cucumber features breaks.

• Update the bundled major version of factory_bot to 5.x.

How to test: none, automated.