Make the default :info flash display "Info" instead, and add a :success flash with the old behaviour

[DitherWither]

Fixes #5795

Also use blue for the info flash, as that is what info dialogues tend to use.

[josevalim]

Thank you for the PR. However, I think we should keep the generated code simple and not add a new style. We should either rename :info to :success or change the style of :info to something slightly more neutral (or keep it all as is).

[mattmatters]

We've done something similar on a couple projects. I think the defaults are fine as they are, but some of the documentation doesn't actually make it very clear that flash types can be whatever (at least in the liveview documentation). Maybe we start with just adding a little nice clarification here for now.

[SteffenDE]

@mattmatters the docs were already adjusted in response to phoenixframework/phoenix_live_view#3342, but that’s not released yet :)