Update PHP version to 8.1 in .env.dist

Bump the default PHP version from 8.0 to 8.1 for local development and testing environments.

Author: josephfusco

.env.dist

@@ -50,5 +50,5 @@ MYSQL_PASSWORD=${DB_PASSWORD}
 
 # Change these to test different versions of WP, WPGraphQL, ACF etc.
 WP_VERSION=6.9
-PHP_VERSION=8.0
+PHP_VERSION=8.1
 WPGRAPHQL_VERSION=latest