PHPC-1504: Add PHP 7.4 to AppVeyor build matrix
PHPC-1504: Add PHP 7.4 to AppVeyor build matrix

This adds x64 and x86 with and without ZTS for PHP 7.4, and reduces PHP 7.3 to x64 with and without ZTS.

mongodb/mongo-php-driver,mongodb/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,jmikola/mongo-php-driver-prototype,mongodb-labs/mongo-php-driver-prototype,mongodb-labs/mongo-php-driver-prototype,jmikola/mongo-php-driver-prototype,jmikola/mongo-php-driver,mongodb/mongo-php-driver,mongodb/mongo-php-driver,jmikola/mongo-php-driver,mongodb/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,jmikola/mongo-php-driver,jmikola/mongo-php-driver,jmikola/mongo-php-driver,jmikola/mongo-php-driver-prototype,jmikola/mongo-php-driver-prototype,jmikola/mongo-php-driver-prototype,mongodb-labs/mongo-php-driver-prototype