acceptance tests - update plan check logic to not error if the resource isn't found in the plan

[stephybun]

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Description

The plan check CheckPlan was introduced by #27319 to enable the acceptance testing shim to catch and error out from update tests that triggered a ForceNew. The current logic in CheckPlancauses a number of acceptance tests in the provider to fail with an error similar to the one below:

------- Stdout: -------
=== RUN   TestAccImage_standaloneImage
=== PAUSE TestAccImage_standaloneImage
=== CONT  TestAccImage_standaloneImage
    testcase.go:173: Step 1/4 error: Pre-apply plan check(s) failed:
        azurerm_image.test - Resource not found in plan ResourceChanges
--- FAIL: TestAccImage_standaloneImage (18.69s)
FAIL

Tests failing with this error rely on a setup configuration as the first test step before applying the test configuration that's pertinent for testing the actual resource.

This PR inverts the logic in CheckPlan to only check for a Replace action and error if the resource exists in the plan as opposed to error'ing if it's missing.

Testing

Ideally the entire testing suite is run to catch any edge cases, given the size and scope of that it's not a feasible undertaking.

I ran three tests to validate the behaviour is as expected:

TestAccImage_standaloneImage - previously failed, now passes
TestAccAppConfigurationKey_basicNoValue - failed because it triggers a ForceNew, still fails meaning the original intent of the check is still intact
TestAccAppConfigurationKey_KVToVault - passed, still passes

[jackofallops]

Thanks @stephybun - This LGTM 🍳