Declare PHP 8.4 supported

- Do it in the PHP policy page: By MDL-80117, since 5.0
- Update 5.0 release notes (not needed in this case, it's
    already showing the correct PHP 8.4 information).

Author: andrewnicols

general/development/policies/php.md

@@ -49,6 +49,12 @@ You must be logged in to tracker to see issues in Epics.
 
 ## PHP supported versions
 
+### PHP 8.4
+
+<Since versions={["5.0"]} issueNumber="MDL-80117" />
+
+PHP 8.4 **can be used with** Moodle 5.0 and later releases. See MDL-80117 for details.
+
 ### PHP 8.3
 
 <Since versions={["4.4"]} issueNumber="MDL-76426" />