Consolidate Eligibility Start templates for older adult, veteran, and CalFresh cardholder into one Login.gov Eligibility Start template

[angela-tran]

...we may want to revisit the idea that each media item needs its own template and/or that the media item templates need to be specific to the eligibility pathway.

Really it seems like the are specific to the eligibility verifier, since all of calfresh, senior, and veteran use the exact same text to describe Login.gov.

Originally posted by @thekaveman in #1962 (review)