Add release 1.18 to metadata.yaml

[willie-yao]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
Adds the impending 1.18 release series to metadata.yaml.

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:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• cherry-pick candidate

Release note:

NONE

[mboersma]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 9f838fbbfb8c230dc0d1daba6dfa2b2c54fa5f7e

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mboersma

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Needs approval from an approver in each of these files:

• OWNERS [mboersma]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.41%. Comparing base (2329218) to head (c7adf26).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5394   +/-   ##
=======================================
  Coverage   52.41%   52.41%           
=======================================
  Files         272      272           
  Lines       29361    29361           
=======================================
  Hits        15390    15390           
  Misses      13165    13165           
  Partials      806      806           

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