Drop deprecated ResourceManager APIs

[scothis]

Drops deprecated APIs for ResourceManager, and migrates tests from the ChildReconciler to the UpdatingObjectManager. This was a holdover from before ResouceManager was extracted from ChildReconciler.

The StubObjectManager is useful for testing a reconciler that needs to interact with an ObjectManager. It will preform a basic create/update/delete and nothing else.

The ObjectManagerReconcilerTestHarness is useful for testing a custom ObjectManager in the context of a reconciler test case. The actual, expected and result objects are stored in the reconciler stash.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 33.33333% with 64 lines in your changes missing coverage. Please review.

Project coverage is 58.93%. Comparing base (72b6e1c) to head (674a20e). Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
testing/objectmanager.go	0.00%	57 Missing :warning:
testing/client.go	33.33%	4 Missing :warning:
testing/subreconciler.go	0.00%	2 Missing :warning:
reconcilers/stash.go	93.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #549 +/- ## ========================================== - Coverage 59.83% 58.93% -0.90% ========================================== Files 32 33 +1 Lines 2958 2981 +23 ========================================== - Hits 1770 1757 -13 - Misses 1094 1130 +36 Partials 94 94 ```

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