[IBAN validator] Fixed and improved handling of lowercase values

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10481
| License       | MIT

The IBAN validator didn't correctly handle strings with lowercase letters. It seemed to accept lowercase values as well, but actually didn't.

This PR introduces an `acceptLowercase` option which on default is `false` to avoid any BC breaks. If you set it to true, IBAN strings containing lowercase letter will also pass if they are otherwise valid.