[9.x] Add uppercase validation rule

[michaelnabil230]

This PR introduces an "uppercase" validation rule.

We have a current want for this rule to create some standardized data formatting without changing the case on the user silently.

For example, we want a user to be able to create a database user, but we want to force the user to uppercase.

Usage:

Validator::make(['name' => 'ADMIN'], ['name' => ['required', 'string', 'uppercase']);

I don't have a real use case for an "uppercase" rule, so I did not include it. If we want it for consistency, I can follow up with one (or someone else is welcome to add it).

Documentation: laravel/docs#8336

[MaRmARk0]

I recently wanted to validate whether incoming language identifier is an uppercase string, e.g. "SK", "CZ", "EN" etc. so this comes handy now.