Extension rule allows wrong files and disallows uppercased extensions

[FrittenKeeZ]

Describe the bug
Given the extension rule ext:jpg,png you're allowed to select example.apng but not example.PNG.

To Reproduce
Steps to reproduce the behavior:

1. Add an image field to the top level of a page
2. Set the validation rule to validate: 'ext:jpg,png'
3. Try to add a example.PNG file with capital extension
4. See that the validation error occurs.

Expected behavior
Regardless of capitalization of extension a file should be allow - and files not matching extension including dot ( .) shouldn't.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment details (please complete the following information):

• Statamic Version: 2.11.20

Additional context
It's a matter of correcting statamic/core/Providers/ValidationServiceProvider.php:97.