Currently, calling `downgradeModule()` more than once is not supported.
If one wants to downgrade multiple Angular modules, they can create a
"super-module" that imports all the rest and downgrade that.
This commit adds support for downgrading multiple Angular modules. If
multiple modules are downgraded, then one must explicitly specify the
downgraded module that each downgraded component or injectable belongs
to, when calling `downgradeComponent()` and `downgradeInjectable()`
respectively.
No modification is needed (i.e. there is no need to specify a module for
downgraded components and injectables), if an app is not using
`downgradeModule()` or if there is only one downgraded Angular module.
Fixes#26062
PR Close#26217
