[release/4][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.

[d2iq-mergebot]

This repo has @mesosphere-mergebot integration. You can perform the following commands by submitting a comment. Submit a comment with content "@mesosphere-mergebot help" to view more detailed help text and examples. Be sure the have a look at the mergebot documentation, too.

@mesosphere-mergebot backport