Remove Kubernetes External Secret from docs

[chaspy]

It seems to be deprecated.
https://github.com/external-secrets/kubernetes-external-secrets

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Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
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• I've updated documentation as required by this PR.
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[codecov]

Codecov Report

Merging #9251 (71baa8d) into master (f99e5b9) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9251      +/-   ##
==========================================
- Coverage   45.73%   45.71%   -0.02%     
==========================================
  Files         220      220              
  Lines       26006    26006              
==========================================
- Hits        11893    11888       -5     
- Misses      12456    12460       +4     
- Partials     1657     1658       +1     

Impacted Files	Coverage Δ
applicationset/services/scm_provider/github.go	75.29% <0.00%> (-5.89%)	⬇️

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[crenshaw-dev]

Thanks!