[v2.9] Improve securityContext for operator Deployment

[mjura]

1. Restrict container from acquiring additional privileges (securityContext.allowPrivilegeEscalation)
2. Mount container's root filesystem as read only (securityContext.readOnlyRootFilesystem)
3. Ensure that container won't be started as privileged container (securityContext.privileged)

Issue: https://github.com/rancher/aks-operator/issues/591 (cherry picked from commit 7bc096f030238b2384c4ed7e5f1ae66ebf23be17)

What this PR does / why we need it:

Which issue(s) this PR fixes Issue #

Special notes for your reviewer:

Checklist:

• [ ] squashed commits into logical changes
• [ ] includes documentation
• [ ] adds unit tests
• [ ] adds or updates e2e tests
• [ ] backport needed