[Migration] Add password validation feature behavioural changes to migration docs

[Thumimku]

Describe the issue:
Related Issue: #19044

With this feature, we introduced organisation wide password validation mechanism where the admin can configure password validation rules which is common for all userstores.

Earlier we had user store wise password validation feature, which not gonna promoted for new users. In our IS 7 doc space we don't mention about userstore level password validation.
But we need to mention this in our migration doc.

We need to mention,

• Whats the new feature (Org wise input validation listener for password)
• The deprecation of user store level password validation and password policy handler
• How to revert it back to old behaviour if necessary.