Add missing OAuthenticatable interface step to upgrade guide

[FeBe95]

The upgrade guide was missing a step instructing users to add the Laravel\Passport\Contracts\OAuthenticatable interface to their App\Models\User model. This interface was introduced in Passport 13. Prior versions only required the HasApiTokens trait. Without this, upgrading users will have no indication they need to add it.

Changes

Added a new entry to UPGRADE.md under the "Upgrading To 13.0 From 12.x" section documenting the required User model change, consistent with what the installation docs already state.

[hafezdivandari]

This is optional, that's why it is not mentioned on the upgrade guide but in the docs for new installations. Upgrade guide usually contains breaking changes only.

[FeBe95]

@taylorotwell Thanks for merging!

@hafezdivandari We were already using the HasApiTokens trait before. The trait now includes the @phpstan-require-implements annotation to make sure that the trait is only used with OAuthenticatable classes. PHPStan started reporting errors because of this change, but I couldn’t find any mention of it in the upgrade guide. That’s why I thought it would be helpful to include it there.