chore: remove cert manager 1.15.0

[cwyl02]

What type of PR is this?

What this PR does/ why we need it:
for #1502

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Checklist

• If a chart is changed, the chart version is correctly incremented.
• 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.