Updates for all angular.io links to the new angular.dev domain. Additionally, adjustment to new resources where the equivalent does not exist on the new site (e.g. Tour of Heroes tutorial)
G3 is now using RXJS version 7 which makes it possible for the CLI to also be updated to RXJS 7.
NB: this change does not remove all usages of the deprecated APIs.
Closes#24371
This should clean up the code a bit.
Note: at first I added the no-useless-cast rule, but after getting frustrated
with it (as it has many false positive), I decided to remove the rule but some
useless casts were removed so I let those in the PR.