[release/3][BACKPORT] chore: Bump prometheus to 15.4.11

[d2iq-mergebot]

This is a backport of the following PR:

#1427

What type of PR is this?

bug

What this PR does/ why we need it:

bumps prom to latest mesosphere/charts#1278 which pins some images to use those already deployed in konvoy/kommander to decrease image bundle

Which issue(s) this PR fixes:

https://jira.d2iq.com/browse/COPS-6963

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

chore: Pins Grafana image to 7.5.6 and bumps kubectl image to 1.20.6 (COPS-6963)

Checklist

• The commit message explains the changes and why are needed.
• The code builds and passes lint/style checks locally.
• The relevant subset of integration tests pass locally.
• The core changes are covered by tests.
• The documentation is updated where needed.
  Before merging, conflicts in the following files need to be resolved:
  addons/prometheus/prometheus.yaml

[d2iq-mergebot]

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