add support for PHP8.0

Fixed for PHP 8 Compatibility

Merge pull request ongr-io#338 from mitelg/rename-class

Rename MatrixStatsAggregation.php to MaxAggregation.php

Rename MatrixStatsAggregation.php to MaxAggregation.php

support serializer v5 and upgrade php minimum requirement to the 7.2

add elasticsearch/elasticsearch to the composer file

update travis

rollback support for v3 and v4 serializer

upgrade elasticsearch version in the travis to the 7.5.2

update to support the symfony serializer 5.0