chore: Fix psalm to use correct php version depending on branch

Needs to be adapted on each branch to reflect the used PHP version and
 help with baseline updating.

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

psalm.xml

@@ -8,6 +8,7 @@
     errorBaseline="build/psalm-baseline.xml"
 	findUnusedBaselineEntry="false"
 	findUnusedCode="false"
+	phpVersion="8.1"
 >
 	<plugins>
 		<plugin filename="build/psalm/AppFrameworkTainter.php" />