[WIP] Deprecate interval in manager task schedule spec

[rzetelskik]

Description of your changes: This PR deprecates the interval field in manager task schedule spec and adjusts the existing tests to the deprecation.

Which issue is resolved by this Pull Request: Resolves https://github.com/scylladb/scylla-operator/issues/1796

/kind feature /priority important-longterm /hold for https://github.com/scylladb/scylla-operator/pull/1851

[scylla-operator-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rzetelskik, zimnx

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[rzetelskik]

known manager flake /test images /retest

[scylla-operator-bot[bot]]

@rzetelskik: The following tests 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/e2e-gke-release-script-latest	804f6fcfb09120c2d595dbf891d31a5c2c74a54f	link	true	/test e2e-gke-release-script-latest
ci/prow/images	9072ededeac37ec30f66259af229a6d3759b34cf	link	true	/test images
ci/prow/e2e-gke-parallel	9072ededeac37ec30f66259af229a6d3759b34cf	link	true	/test e2e-gke-parallel

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