Run stan checks on PHP 7.3

This prevents phpstan from complaining about new setcookie() signature.

.github/workflows/ci.yml

@@ -110,11 +110,11 @@ jobs:
     - name: Setup PHP
       run: |
         # Without removing php-fpm extensions installation fails for some reason
-        sudo apt remove php7.2-fpm
-        sudo apt install php7.2-intl
-        sudo update-alternatives --set php /usr/bin/php7.2
-        sudo update-alternatives --set phar /usr/bin/phar7.2
-        sudo update-alternatives --set phar.phar /usr/bin/phar.phar7.2
+        sudo apt remove php7.3-fpm
+        sudo apt install php7.3-intl
+        sudo update-alternatives --set php /usr/bin/php7.3
+        sudo update-alternatives --set phar /usr/bin/phar7.3
+        sudo update-alternatives --set phar.phar /usr/bin/phar.phar7.3
         sudo phpdismod xdebug
 
     - name: composer install