Update docs for upgrading to freeform 5 RE customAttributes

[iamtompickering]

What happened?

Had to dive into the plugin files to figure it out, but we had a number of variables getting custom attributes e.g. form.customAttributes.overrideValues['heading'] and it looks like this needs to be updated to form.getProperties.get('overrideValues').heading.

Might be worth adding this to your key notes for upgrading.

Errors and Stack Trace (if available)

No response

How can we reproduce this?

1. Upgrade from Craft 4 > 5

Freeform Edition

Pro

Freeform Version

5.8.0

Craft Version

5.5.5

When did this issue start?

After upgrading from older Freeform version, After upgrading from older Craft version

Previous Freeform Version

No response