ci(psalm): Define PHP 8.1 as minimum

Signed-off-by: Joas Schilling <coding@schilljs.com>

psalm.xml

@@ -7,6 +7,7 @@
     errorLevel="2"
     findUnusedBaselineEntry="false"
     findUnusedCode="false"
+    phpVersion="8.1"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xmlns="https://getpsalm.org/schema/config"
     xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"