Warn about forms with old version of entity spec <2024.*

[ktuite]

When we migrate the entity form spec in existing entity forms we will not want to accept want to warn about new forms with an old entity spec.

From the above issue:

Once forms are migrated, we will need to check going forward that all form definitions that are uploaded (for new or existing forms) are capable of making offline changes. Specifically, we should reject form definitions that specify an entities spec before 2024.1. That shouldn't be an issue for users using XLSForms (things should work seamlessly), because pyxform will automatically specify 2024.1.

UPDATE: The issue mentioned rejecting form definitions, but we've decided to show a warning about them instead.

One hurdle users might encounter is if they have an old version of their form in XML and it has an old entity spec version. In that case, we can give users info on how to recover (e.g. download upgraded XML, look at spec to figure out changes to apply).

[matthew-white]

We've discussed that it'd be nice not to outright reject forms like this if doing so is not necessary. Instead, we can warn users if the form definition they've uploaded specifies an old spec. I'll update the description above.

Surfacing the warning in Frontend will probably require changes to Frontend. I've added the frontend label to the issue.

I've also removed the documentation label, since it isn't a docs issue. There's an issue at getodk/docs#1855 to update user docs, and an issue at getodk/central-backend#1211 to update API docs. Feel free to edit those!