Remove kubectl image tag override

Summary

!450 (merged) added an override for the kubectl image tag, specifying a digest that contained the throttling fix found in gitlab-org/charts/gitlab#3201 (closed).

Because the fix was cherry-picked into 5-9-stable with gitlab-org/charts/gitlab!2508 (merged), once all the CHART_VERSIONS are 5.9.4 and above, we can remove this fix.

Acceptance criteria

• All CHART_VERSIONS are >= 5.9.4
• kubectl image tag override is removed