allow newer composer dependencies

[davidcoutadeur]

Currently, composer dependencies are limited to platform php 7.4

in composer.json:

    "config": {
        "platform": {
            "php": "7.4"
        }
    }

We should benefit from newer composer dependencies by removing this limit.

[coudot]

This was linked to #897

[davidcoutadeur]

The main problem is that we currently provide a common tarball that is the source for all operating systems / versions of php. This tarball is also used for building the rpm and deb packages.

This is a actually a limitation, preventing us from having the last dependencies. On the other hand, having different versions for some dependencies in the final packages can lead to untested dependencies, and this in incompatibilities.

Two ways for managing this:

• provide multiple tarballs: one for each distribution, with the appropriate dependencies updated
• provide a unique tarball without the dependencies. The rpm/deb packaging system must then pull these dependencies at build time (composer update)

This require structural changes in the release process, and must be correctly evaluated.