Fix outdated documentation for ReflectionClass::setStaticPropertyValue visibility handling

[lacatoire]

Issue 4600 The documentation for ReflectionClass::setStaticPropertyValue() is outdated.

Since PHP 7.4.9, this method can set static properties regardless of their visibility (public, protected, or private), which is covered by core tests. The current description still states that private and protected properties would fail.

This PR updates:

• the short description and refpurpose,
• removes the outdated visibility limitation,
• and adds a changelog entry for PHP 7.4.9 while keeping the historical 7.4.0 note.