bump prometheus-operator to use forked chart

[joejulian]

What type of PR is this?

chore

What this PR does/ why we need it:

The upstream helm chart has been deprecated. This uses our fork of that chart.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

none

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.