[Sync EN] Add PHP 8.4.0 changelog entries for Intl and MBString pages (#5365)

Commit: `https://github.com/php/doc-en/commit/1be57c2d7d7306b6167a4305059fe1abe8911699`
PR: `https://github.com/php/doc-en/pull/5365`

**Archivos ES a actualizar**
- `reference/intl/idn/idn-to-ascii.xml`
- `reference/intl/idn/idn-to-utf8.xml`
- `reference/mbstring/functions/mb-strcut.xml`
- `reference/mbstring/functions/mb-substr.xml`

