add CHANGELOG for v1.20

[alimaazamat]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
Adds a changelog file for the impending CAPZ v1.20.0 minor release

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

[willie-yao]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: f8f50eeda84f71813abbdb460e18eafbe4185311

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: willie-yao

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Needs approval from an approver in each of these files:

• OWNERS [willie-yao]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.83%. Comparing base (72e27d0) to head (7fd60a2).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5694      +/-   ##
==========================================
- Coverage   52.84%   52.83%   -0.01%     
==========================================
  Files         278      278              
  Lines       29610    29610              
==========================================
- Hits        15647    15645       -2     
- Misses      13146    13148       +2     
  Partials      817      817              

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