Decision on deprecation and removal of old PHP/NC versions

[christianlupus]

There are clear end-of-life dates for both PHP and Nextcloud. We currently support more versions than officially supported (NC since 19 ATM and PHP since 7.3/7.4).

During my work on #1093, I found that our tests were not stable. Adding the composer.lock to git enhanced that. However, it is not trivial to have one composer.lock for all supported version combinations. There can be inconsistencies introduced. As a result, we might run sooner or later into the situation where we will have to drop support for older PHP versions or NC releases.

Additionally, the more versions we support, the more testing is involved. This is also true for the automated tests in GitHub. ATM there are 12 complete runs of the test suite required. If we could drop some versions, that would speed up development eventually as well. I had to drop support for PHP 7.3.

We should consider removing older (read EOL) versions from our support list. We should have a community understanding of when a version will no longer be supported and how to prepare such removal (something like a deprecation notice?).

This might be related to #830 as with that we could have a clearer image of our user base and their system configurations.