update to PHP 8 & ES 8

.gitignore

.travis.yml

Makefile

README.md

composer.json

phpunit.xml.dist

src/Aggregation/AbstractAggregation.php

src/Aggregation/Bucketing/AdjacencyMatrixAggregation.php

src/Aggregation/Bucketing/AutoDateHistogramAggregation.php

src/Aggregation/Bucketing/ChildrenAggregation.php

src/Aggregation/Bucketing/CompositeAggregation.php

src/Aggregation/Bucketing/DateHistogramAggregation.php

src/Aggregation/Bucketing/DateRangeAggregation.php

src/Aggregation/Bucketing/DiversifiedSamplerAggregation.php

src/Aggregation/Bucketing/FilterAggregation.php

src/Aggregation/Bucketing/FiltersAggregation.php

src/Aggregation/Bucketing/GeoDistanceAggregation.php

src/Aggregation/Bucketing/GeoHashGridAggregation.php

src/Aggregation/Bucketing/GlobalAggregation.php

src/Aggregation/Bucketing/HistogramAggregation.php