chore: target PHP 8.4 in psalm config

Set phpVersion="8.4" so Psalm targets 8.4 explicitly instead of
inferring 8.3 from composer.json's ^8.3|^8.4 constraint.

psalm.xml

@@ -4,6 +4,7 @@
     xmlns="https://getpsalm.org/schema/config"
     xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
     errorLevel="8"
+    phpVersion="8.4"
     findUnusedCode="false"
     ensureOverrideAttribute="false"
     errorBaseline="psalm-baseline.xml"