Validate that containers run as non-root

[AleksandarSavchev]

How to categorize this PR?

/area security quality
/kind enhancement

What this PR does / why we need it:
This PR enhances the securityContext of pods that already run as non-root with the runAsNonRoot field set to true. This will force the kubelet to validate that the containers in a pod are started with a non-root user.

This PR also modifies fluent-operator to run as nonroot user & group 65532 (id of nonroot user in distroless images ref). Aiming to adhere to the principles of least privilege and reduce potential attack surface.

Which issue(s) this PR fixes:
Fixes #7157

Special notes for your reviewer:

Release note:

Containers, configured to run as `non-root`, are now validated to start with `non-root` user by the `kubelet`.

The `fluent-operator` component now runs as `nonroot` user and group `65532`.

[rfranzke]

/lgtm

[gardener-prow]

LGTM label has been added.

Git tree hash: 4fcdc376ac633feefd068cd324f2e4d0b7f70eb4

[oliver-goetz]

Thanks 😄
/approve

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: oliver-goetz

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[gardener-prow]

@AleksandarSavchev: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gardener-e2e-kind	545d98b	link	unknown	/test pull-gardener-e2e-kind

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