fix(chart): fixes a compatibility issue with the Helm Chart and Prometheus v3.0+

[cdoble84-uk]

In the Flagger Helm Chart Prometheus config the storage retention duration is set via this flag --storage.tsdb.retention. This Flag was deprecated in a previous version of Prometheus and removed from Prometheus v3.0+ which means the current Flagger Helm Chart is incompatible with the newer Prometheus versions.

This flag has now been updated to --storage.tsdb.retention.time which is backwards compatible down to Prometheus v2.51 at least.