fix: clear outdated_helm_chart for charts no longer referenced

[Kitio-Tek]

Fixes #2030

reconcileOutdatedHelmChartMetric carried forward every outdated chart that was not checked in the current pass. That is meant for a chart whose repository could not be fetched, but it also caught a chart that no ClusterSummary references anymore: its ClusterSummary was deleted, or the chart was removed from the profile. Such a chart is never checked again, so its outdated_helm_chart series stayed at 1 until the addon-controller restarted.

The pass now records the charts it collects from the ClusterSummaries (markReferenced), and reconcileOutdatedHelmChartMetric carries forward only an unchecked chart that is still referenced. A chart whose fetch failed keeps its series, as before. I also fixed the comment on outdatedHelmChartGauge: it said the gauge is reset on every pass, which the checker deliberately doesn't do.

The tests in helmchart_outdated_check_test.go run a whole checkOutdatedHelmCharts pass against a fake client, after a pass that found the chart outdated:

• the ClusterSummary was deleted: the series is gone;
• the chart was removed from the profile: the series is gone;
• the repository can't be reached: the series stays;
• the chart is up to date, from an index served by httptest: the series is gone.

make test and make lint pass.

@gianlucam76 this follows the direction from the issue.