[8.x] Copy password rule to current_password

[netpok]

This PR does not contain breaking changes.

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With the introduction of the new password rule we have to different rule under the same name:

• The Password class: which validates new passwords
• The password rule: which validates the current password

This creates communication problems where the referenced rule can only be selected by the application context.

This PR moves the password validation to current_password and adds an alias to keep backwards compatibility. This allows:

• All existing code function like nothing happened
• All new code to use the new rule
• Update existing projects when Laravel 9 is released

While not part of the PR, the main intention is to completely remove the password rule from the next major release (and maybe use it as an alias to Password::default() later in Laravel 10).

If this PR accepted I will create others to add the necessary translation lines to the laravel/laravel repository and the necessary documentation update to laravel/docs.