Bump mongoid from 7.1.2 to 7.3.1
Bump mongoid from 7.1.2 to 7.3.1

Bumping `mongoid` to version 7.3.1 to remove the requirement for
`activemodel (>= 5.1, < 6.1)` to `activemodel (>= 5.1, < 6.2)`.

We cannot yet upgrade to 7.3.2 as this has a dependency on
`ruby2_keywords (~> 0.0.5)`.

alphagov/publisher,alphagov/publisher,alphagov/publisher