Use "KB" everywhere instead of "kB"

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Continuation of #10661.

[PHP interprets "kilobytes", "megabytes" etc. in php.ini as having base 2](http://www.php.net/manual/en/faq.using.php#faq.using.shorthandbytes). Therefore, it makes sense to use the same interpretation of these measures in Symfony:

Short form | Long form | Value in Bytes
--- | --- | ---
B | bytes | 1
KB | kilobytes | 1024
MB | megabytes | 1048576
GB | gigabytes | 1073741824

FileValidator, UploadedFile and the ProgressBar helper were changed accordingly.