Rename MatrixStatsAggregation.php to MaxAggregation.php

[jdreesen]

Cherry pick of #313 for Elasticsearch 6 and Symfony 5

[jdreesen]

@84m Would you please cut a new release 6.2.x release?

[codecov-io]

Codecov Report

Merging #339 into 6.2 will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                6.2     #339   +/-   ##
=========================================
  Coverage     78.53%   78.53%           
  Complexity      713      713           
=========================================
  Files           123      123           
  Lines          1887     1887           
=========================================
  Hits           1482     1482           
  Misses          405      405           

Impacted Files	Coverage Δ	Complexity Δ
src/Aggregation/Matrix/MaxAggregation.php	0.00% <ø> (ø)	10.00 <0.00> (?)

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