Clarify authentication settings to Kibana related to #4495

[m-a-leclercq]

Removing mentions of kibana_user and kibana_password since #4495 removed them entirely.

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Summary - What I changed

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• Added a label for the type of pr: bug, enhancement, schema, maintenance, Rule: New, Rule: Deprecation, Rule: Tuning, Hunt: New, or Hunt: Tuning so guidelines can be generated
• Added the meta:rapid-merge label if planning to merge within 24 hours
• Secret and sensitive material has been managed correctly
• Automated testing was updated or added to match the most common scenarios
• Documentation and comments were added for features that require explanation

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