'Gen' filters on Starter Select are not the same as the 'Mono Gen' Challenge filters

[damocleas]

As of #5125 , it is not as easy for new players to understand the missing pokemon.

This could likely be resolved by having the filter work in a similar way to the Challenge Mode filter (without being greyed out) and still be understandable to new players.

Gen 1 filter on Starter Select

Challenge Mode - Mono Gen 1 - Starter Select

This can make later gens look a bit unexpected but it should be understandable

Gen 8 filter on Starter Select

Challenge Mode - Mono Gen 9 - Starter Select