Timestamps check should ignore ActiveStorage table

[bmulholland]

On latest main:

add a created_at column to active_storage_variant_records
add a updated_at column to active_storage_variant_records

These are created/managed by ActiveStorage: https://guides.rubyonrails.org/active_storage_overview.html#setup

So should be ignored out of the box for a good developer experience.

[jdufresne]

Maybe Active Record Doctor could ignore third party models and tables more generally. I experience a similar warning and so have globally ignored these tables. But other third party gems have similar issues.