Add a note about vulnerability reporting to the contributor guide

[BoHristova]

How to categorize this PR?

/kind task

What this PR does / why we need it:

Added a note to the Contributor Guide warning contributors against opening public issues for security vulnerabilities.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
/cc @HeckEK

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[coderabbitai]

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[gardener-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[github-actions]

Sitemap structure diff vs master

	count
➕ added pages	0
➖ removed pages	0

First entries (full diff in the run summary & artifact):

ADDED   (0):
  (none)
REMOVED (0):
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A rename appears as one removed (old path) + one added (new path).