improve renovate config and track only the minor version tag for kubectl

General MR

Summary

• Track minor version updates rather than patch updates to the kubectl image.
• Use renovate's built-in helm-values manager rather than custom regexes.

Relevant logs/screenshots

update-everything

library-subcharts-only

app-image

other-image

chart-only

chart-and-image

Linked Issue

Closes #290 (closed).

Upgrade Notices

N/A