Change osImageURL to be a required field in docs

[rahulbabu95]

Description of changes:
In release 0.19 we changed the osImageURL field in our API to be a required field from optional. And in release-0.20 we changed the osImageURL field on TinkerbellMachineConfigs also to be a required field. Call these changes out on the docs.

Testing (if applicable):

Documentation added/planned (if applicable):

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.48%. Comparing base (ebcd518) to head (9bbb644).

❗ Current head 9bbb644 differs from pull request most recent head 125baad

Please upload reports for the commit 125baad to get more accurate results.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8414   +/-   ##
=======================================
  Coverage   73.48%   73.48%           
=======================================
  Files         578      578           
  Lines       36489    36489           
=======================================
  Hits        26814    26814           
  Misses       7956     7956           
  Partials     1719     1719           

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[rahulbabu95]

/approve

[eks-distro-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rahulbabu95

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[rahulbabu95]

/cherrypick release-0.20

[eks-distro-pr-bot]

@rahulbabu95: new pull request created: #8415

In response to this:

/cherrypick release-0.20

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