Set the backlog metrics interval through the generic override

[delthas]

backbeat.backlogMetricsIntervalSeconds is a dedicated CR field the operator no longer has: scality/zenko-operator#626 added it, scality/zenko-operator#633 reverted it right after v1.8.16 in favour of the generic backbeat.configurationOverrides map.

The two spellings never coexist in a released operator:

operator	backlogMetricsIntervalSeconds	backbeat.configurationOverrides
v1.8.16	works	absent — the field exists only on ZenkoSpecSorbet
v1.8.17	removed	works

So the config and the operator pin have to move together, and bumping the operator without this change fails the whole suite on a rejected field. Hence both in one commit.

KAFKA_BACKLOG_METRICS_INTERVALS is backbeat's declared env alias for kafka.backlogMetrics.intervalS (intervalS: joi.number().default(60).meta({ env: 'INTERVALS' })), so the value reaches the same key with the same unit — seconds — as the removed field did. It is set on the four consumers whose lag the lifecycle conductor waits on.

Split out of #2532, which carries the same change but is blocked on a zenko-operator 1.9 PR and so cannot land on this timeline.

Issue: ZENKO-5359

[bert-e]

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