add release 1.21 to CAPZ metadata

[jackfrancis]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

This PR gets us ready to cut v1.21.0.

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

[nojnhuh]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 640b2de8f30aa67f6f4ae6aac5efdf949831ef8a

[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]

Approvers can indicate their approval by writing /approve in a comment
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.06%. Comparing base (a171336) to head (41854e6).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5818   +/-   ##
=======================================
  Coverage   53.06%   53.06%           
=======================================
  Files         279      279           
  Lines       29677    29678    +1     
=======================================
+ Hits        15747    15750    +3     
+ Misses      13114    13112    -2     
  Partials      816      816           

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