Previously, [UpgradeModule#bootstrap()][1], while being a replacement
for and accepting the same arguments as [angular.bootstrap()][2], did
not return the same value as `angular.bootstrap()` (i.e. the AngularJS
injector in most cases). This made it less straight forward to migrate
some usecases that relied on the return value of `.bootstrap()`. The
work-around was to access the injector via [UpgradeModule#$injector][3]
(after the app had been bootstrapped with `UpgradeModule#bootstrap()`).
This commit addresses this by ensuring `UpgradeModule#bootstrap()`
returns the same value as `angular.bootstrap()`, making it easier to
replace the latter with the former.
[1]: https://angular.io/api/upgrade/static/UpgradeModule#bootstrap
[2]: https://docs.angularjs.org/api/ng/function/angular.bootstrap
[3]: https://angular.io/api/upgrade/static/UpgradeModule#%24injectorFixes#46211
PR Close#46214