Support browser engines, not specific browsers

### Your use case

#### What would you like to do?
Right now, Element only supports the latest versions of Firefox, Chrome, and Edge. And it tells people to use those when they're using something else.

2 of the 3 choices are actively hostile to user privacy (Chrome and Edge), whereas Firefox is relatively neutral (but unfortunately leaves some defaults as non-private). The warning has the effect of pushing people to use these
mostly user-hostile browsers.

I understand not being able to test on all the different browsers, but what tends to matter for rendering is the rendering engine rather than the chrome around it.

It'd be great if, instead of specifying supported browser versions, Element would support browser engine versions instead. And therefore warned only about browsers that don't use those browser engine versions.

#### Why would you like to do it?
To not discourage people from using privacy-friendly browsers.

#### How would you like to achieve it?
Change the logic behind the browser warning to look at the browser engine instead. And change the relevant documentation too.

### Have you considered any alternatives?

An alternative could be adding more privacy-friendly browsers to the supported browser list, but that seems less ideal.

Or the warning could be reworded to say simply that one might expect strange behavior as the browser is not supported, rather than outright telling people to switch to less privacy-friendly browsers.

### Additional context

_No response_