deflate_* functions were only added on PHP 7

[Luc45]

The CompressGzipstream uses some functions that were only added on PHP 7:

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FOUND 6 ERRORS AFFECTING 3 LINES
----------------------------------------------------------------------
 1476 | ERROR | The function deflate_init() is not present in PHP
      |       | version 5.6 or earlier
      |       | (PHPCompatibility.FunctionUse.NewFunctions.deflate_initFound)
 1476 | ERROR | The constant "ZLIB_ENCODING_GZIP" is not present in
      |       | PHP version 5.6 or earlier
      |       | (PHPCompatibility.Constants.NewConstants.zlib_encoding_gzipFound)
 1483 | ERROR | The function deflate_add() is not present in PHP
      |       | version 5.6 or earlier
      |       | (PHPCompatibility.FunctionUse.NewFunctions.deflate_addFound)
 1483 | ERROR | The constant "ZLIB_NO_FLUSH" is not present in PHP
      |       | version 5.6 or earlier
      |       | (PHPCompatibility.Constants.NewConstants.zlib_no_flushFound)
 1492 | ERROR | The function deflate_add() is not present in PHP
      |       | version 5.6 or earlier
      |       | (PHPCompatibility.FunctionUse.NewFunctions.deflate_addFound)
 1492 | ERROR | The constant "ZLIB_FINISH" is not present in PHP
      |       | version 5.6 or earlier
      |       | (PHPCompatibility.Constants.NewConstants.zlib_finishFound)
----------------------------------------------------------------------

• https://github.com/ifsnop/mysqldump-php/blob/master/src/Ifsnop/Mysqldump/Mysqldump.php#L1473
• https://www.php.net/manual/en/function.deflate-init.php

[ifsnop]

I think we should better rollback this commit. I appreciate backwards compatibility.