Add installation/upgrade warning in MODX 3 installer

[Ruslan-Aleev]

Bug report

Summary

Very often on forums, chats, users report that when updating MODX from 2.x branch to 3.x, the site stops working correctly or even crashes.

Maybe add a warning that COMPONENTS ARE NOT ADAPTED, and don't update MODX unless you're sure about it?

Maybe add a check in the installer that would prohibit upgrading from MODX 2.8 to MODX 3, and then remove it (for example, for version 3.3, when a significant % of the components will be adapted)?

Environment

MODX 3

[Ruslan-Aleev]

Related with #16024

[JoshuaLuckers]

It's almost impossible to determine compatibility issues once you're in the actual upgrade process.

The first step of the upgrade process starts when you copy the new files into your existing installation (this overwrites the old files), this means the "newer" version of MODX is being used from now on.
If there are compatibility issues with extra's, plugins, other custom stuff, etc things might/will break from this point forward.

[Ruslan-Aleev]

It's almost impossible to determine compatibility issues once you're in the actual upgrade process.

Then the only option is to write an informational message in the installer.