[€50 bounty] Updating PHP dependencies: Slim, dotenv, phpunit

[Mark-H]

As can be witnessed in the closed pull requests from Dependabot linked below, we're a little behind on a bunch of PHP dependencies. This includes major updates with breaking changes that require code changes.

Help with this would be appreciated.

• Slim 3 to 4 https://www.slimframework.com/docs/v4/start/upgrade.html
• dotenv 3 to 5 https://github.com/vlucas/phpdotenv/blob/v5.0.0/UPGRADING.md#v3-to-v4, https://github.com/vlucas/phpdotenv/blob/v5.0.0/UPGRADING.md#v4-to-v5
• phpunit 7 to 9 - low prio because there's not much of a test suite to talk off

[Mark-H]

There's a €50 bounty on getting these packages upgrade.