update fit registration mailer not to be year-specific

[clareconstantine]

(merge after #26276)

This mailer doesn't need to know what year it is, and this will save us having to update it every year.

[mehalshah]

Is there a test somewhere that invokes this mailer?

[clareconstantine]

Update FiT Registration for 1920

[clareconstantine]

@mehalshah there isn't currently a test, just the mail previews and their test, which iterates over all previews and doesn't refer to mailers by class name.