Fix test matrix for "master" (php version needs to be 7.4)

[gthomas2]

Tests currently failing on master due to composer install fail:

Error: Your lock file does not contain a compatible set of packages. Ple
ase run composer update.%0A%0A Problem 1%0A - Root composer.json requir
es php >=7.4.0 but your php version (7.3.33) does not satisfy that requirem
ent.%0A Problem 2%0A - psr/container is locked to version 1.1.2 and an
update of this package was not requested.%0A - psr/container 1.1.2 requi
res php >=7.4.0 -> your php version (7.3.33) does not satisfy that requirem
ent.%0A Problem 3%0A - psr/container 1.1.2 requires php >=7.4.0 -> your
php version (7.3.33) does not satisfy that requirement.%0A - symfony/se
rvice-contracts v2.5.1 requires psr/container ^1.1 -> satisfiable by psr/co
ntainer[1.1.2].%0A - symfony/service-contracts is locked to version v2.5
.1 and an update of this package was not requested.%0A