Update allowSharingAppStoreAccount deprecation message

[fire-at-will]

Description

Updates the deprecation message of allowSharingAppStoreAccount to tell people who are already using the field to continue using it and not remove it.

[aboedo]

thanks for taking care of this so fast!

[jamesrb1]

Seems like an odd caveat, and worthy of more explanation somewhere? And customers will have to live with the warning message.

Approving so you are not blocked.

[aboedo]

@jamesrb1 this was already true before this PR, it's actually been this way for > 4 years.

So this PR is more surfacing a caveat that has implicitly been there for 4 years and making it explicit, rather than changing anything with regards to how things behave.

It actually came up yesterday when a customer ran into this, so we figured that we should at least improve the error message and related docs. It's very uncommon for it to come up, since the related feature has been deprecated for a very long time, but it's not impossible, so this is meant as a small improvement over the current status