docs: upgrade Angular guide

[arturovt]

I've removed any mentions of Angular versions older than 15 since they haven't been maintained for several years. Angular 15 LTS support will end in May 2024.

I've also eliminated the manual installation guide targeting Angular 6 and older versions from the documentation. It shouldn't be included.

Additionally, I've added a warning about the importance of updating the builder before running schematics.

[MilanKovacic]

Can we still retain the documentation for older Angular versions, but make them less visible? A lot of consumers that are on very old Angular versions use single-spa to gradually migrate/rewrite applications to either new versions of Angular, or other frameworks.

[internettrans]

I agree that keeping the documentation for older versions of Angular is best - some projects are stuck on old versions of Angular and hope single-spa will help them convert the monolith into a microfrontend so they can start new microfrontends with an updated framework.

[arturovt]

@MilanKovacic can you have a look again