We have now support in profile::mediawiki::php for running multiple versions of php alongside each other.
We need to test if moving to php 7.4 would break dumps generation; the idea would be to do so in deployment-prep. Adding a new php version can be done by adding the profile::mediawiki::php::php_version hiera key with an array of php versions *stringified* as values.
So for instance:
profile::mediawiki::php::php_version: ["7.2", "7.4"] # note the double quotes, thanks yaml.
By default the php executable will be linked to the first version in the list.