Add release v1.15 to metadata.yaml

[willie-yao]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

NONE

[mboersma]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: cdc4d0e943cc95d3ba63c34d0c96ce50d63fd983

[nojnhuh]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nojnhuh

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Needs approval from an approver in each of these files:

• OWNERS [nojnhuh]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.00%. Comparing base (392f7d6) to head (63448da).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4821   +/-   ##
=======================================
  Coverage   62.00%   62.00%           
=======================================
  Files         201      201           
  Lines       16857    16857           
=======================================
  Hits        10453    10453           
  Misses       5620     5620           
  Partials      784      784           

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