Make POT creation date static in messages.pot

[rebecca-shoptaw]

Describe the problem that you'd like solved

Now that we've merged in the messages.pot auto-extraction process (#8900), we want to make sure to avoid any contributors who find themselves with auto-added messages.pot updates from having to deal with any merge conflicts as result.

Conflicts with messages.pot will be inevitable if contributors are changing or removing existing text from the site, but they should not occur if contributors are merely adding new text to the site. This is currently happening because of the POT-Creation-Date field which updates when any changes are made.

Proposal & Constraints

While we cannot remove the creation date from the template, we can set it to a fixed date. Demo implementation from the Babel docs below:

from babel.dates import UTC
from datetime import datetime
created = datetime(1990, 4, 1, 15, 30, tzinfo=UTC)
catalog = Catalog(project='Foobar', version='1.0',
                  creation_date=created)

I propose we set the date to be fixed at the last time a new master messages.pot was manually generated, which will be 05/01/24 for now. We can add a comment to the generating function to explain how this date was selected and when to update it.

Additional context

Stakeholders

@cdrini