v1.7.3

This is a minor release to address a change in the Mapbox API, and the way the Mapbox API key is handled (Thanks to @marc2016).

⚠️ When updating to 1.7.3, you now need to provide your own Mapbox API key in the .env file for the event invitation mail static map to work correctly.

Also:
[Dependencies] Update Symfony and other composer dependencies