Disable custom, now obsolete, cherry-pick workflow

[petersutter]

What this PR does / why we need it:
This PR disables the custom cherry-pick workflow, as prow will take over this task (using the same known cherry-pick command)

For example:

• custom cherry-pick PR: [hotfix-1.83] Automated cherry pick of #2719 #2721
• prow cherry-pick PR: [hotfix-1.83] fix: redact authorization headers in Octokit debug logs #2720

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

[gardener-prow]

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