Update doc with instructions to update the migration timestamp

What does this MR do and why?

We have a dangerbot warning about the outdated migrations, but we don't offer instructions to safely and quickly update the migration timestamp.

:hourglass: Migration Timestamp Out of Date
 The following migrations have timestamps that are over three weeks old:
db/migrate/20230609091711_add_max_yaml_size_to_application_settings.rb
Please double check the timestamps and update them if possible. Why does this matter?

This MR adds steps to update timestamps. These steps might be pretty straight forward (maybe even unnecessary) to some, but I believe its better to give the steps since we give a warning.

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