Validate birthday for secured invoice for Shopware versions <6.6

[alexschwarz89]

Problem: If a user enters an invalid birthdate (which is required for secured invoice) the request is sent and declined, for the customer, the order is failed.

Expectation: Validate the birthdate prior sending the actual payment request. I guess this was done in this commit 98267f5, but it's for the 6.6 plugin version. Due to the fact that Shopware has long term support in older versions, is it possible to backport this for the 6.4/6.5 plugin versions as well?

Environment:

Shopware 6.5.8.11
PayOne 5.0.2 (GitHub source)