fix(pg): Restoring behavior for backup scheduling

[sbosio]

Description

When migrating the pb:backups:schedule command, a refactor incorrectly removed the ability to specify timezones through IANA identifiers. The refactor done considered only valid the US timezone abbreviations, when previously they were only used to map them to their IANA equivalents.

Here we restore that functionality and improve the regular expression used to match the scheduled time and zone format, only allowing values for hours between 0 and 23 (previously it would accept the values 24-29, too, which caused a 500 error on the backend).

Test procedure

• Fetch the branch and run yarn and yarn build.
• Create (or reuse) an essential-tier database on any test app you own, e.g. heroku addons:create heroku-postgresql:essential-0 -a <test-app>.
• Run heroku pg:backups:schedule <addon-name> --at "03:00 America/New_York" -a <test-app>, you should see a message that it will default to UTC because the timezone doesn't exist.
• Run the same with the fixed version ./bin/run pg:backups:schedule <addon-name> --at "03:00 America/New_York" -a <test-app>, it should succeed without warnings.

SOC2 Compliance

GUS Work Item

[k80bowman]

Works great, approving but I do have one request. Could you add a test or two for this? I want to make sure we don't mess this up again in the future. Thank you!