Updating tests to validate against PHP version 8.1

Author: snake14

.travis.yml

@@ -42,7 +42,7 @@ matrix:
     - php: 7.2
       env: TEST_SUITE=PluginTests MYSQL_ADAPTER=PDO_MYSQL TEST_AGAINST_CORE=minimum_required_piwik
   include:
-    - php: 8.0
+    - php: 8.1
       env: TEST_SUITE=PluginTests MYSQL_ADAPTER=PDO_MYSQL TEST_AGAINST_PIWIK_BRANCH=$PIWIK_TEST_TARGET SKIP_COMPOSER_INSTALL=1
       script:
         - cd $PIWIK_ROOT_DIR