Add ability to show a warning banner by configuring CheCluster CR

[akurinnoy]

What does this PR do?

The Dashboard looks into the CheCluster CR for a warning message:

spec:
  dashboard:
    warning: "Eclipse Che hosted by Red Hat is going to ..."

and shows it as a permanent banner.

Screenshots

What issues does this PR fix or reference?

eclipse-che/che#20724

[github-actions]

Docker image build succeeded: quay.io/eclipse/che-dashboard:pr-466

[openshift-ci]

@akurinnoy: 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
ci/prow/v8-dashboard-happy-path	0b7dd95	link	true	/test v8-dashboard-happy-path

Full PR test history. Your PR dashboard.

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[olexii4]

LGTM

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: akurinnoy, olexii4

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