Incorrect code is generated when multibyte characters are used

Image Captcha generates incorrect codes when using multibyte characters such as Japanese.

* Example:
- Characters to use in the code: あかさたなは
- Code length: 5
- Generated code: あかさあかさあかさたなはあかさ
- Desirable code: はたかなさ

This issue is caused by issue #3184909, “Fix coding standard for php strlen and substr”.
https://www.drupal.org/project/captcha/issues/3184909 →

This issue occurs in versions 8.x-1.7 through the latest version 2.0.7.

The code is correct before the patch was applied. Using a multibyte function for single-byte processing logic is incorrect. It is working correctly in version 7.x.

Please revert back to the pre-patch version.