Refactor RolloutManager status update logic to eliminate/reduce failed status update errors

[jgwest]

What does this PR do / why we need it:

• Ensures that we only ever update .status field of RolloutManager from a single location in the code
  • We do this by refactoring reconcile logic to return a 'reconcileStatusResult', which indicates what values to set on the status.
• This allows us to avoid these 'status errors we were seeing when running E2E tests: 2024-04-10T05:51:36Z ERROR rollouts-controller unable to update status of RolloutManager {"error": "Operation cannot be fulfilled on rolloutmanagers.argoproj.io \"basic-rollouts-manager\": the object has been modified; please apply your changes to the latest version and try again"}
• This PR also adds a quick Namespace-deletion test, to allow us to prevent modification to resources in a Namespace that is being deleted (which lets us avoid nasty error messages in our E2E tests related to deletion)
  • This allows us to avoid these errors: unable to create new content in namespace argo-rollouts because it is being terminated

Have you updated the necessary documentation?

• Documentation update is required by this PR, and has been updated.

Which issue(s) this PR fixes:
Fixes #?

How to test changes / Special notes to the reviewer:

[jparsai]

All changes LGTM, though at some places error is still thrown while updating resource, but frequency of this is less now.

[jgwest]

Thanks @jparsai for reviewing!