Update to oik v4.15.3 for a couple of security fixes. Tested with WordPress 6.8.2

Changes
| Change | Reference |
|---|---|
| Security fixes: XSS in check theme. XSRF in add theme | Props: Nguyen Xuan Chien |
Tested
- Tested: With WordPress 6.8.2 and WordPress Multisite
- Tested: With PHP 8.3 and PHP 8.4
- Tested: With PHPUnit 9, 10 & 11
