[BUG] Add supported operating system to the tamper protection page [classic]

[joepeeples]

Contributes to #4741 by adding an OS support statement to the classic docs.

Preview:

• Prevent Agent uninstallation

Twin PR for serverless docs:

• https://github.com/elastic/staging-serverless-security-docs/pull/286

[github-actions]

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[pierrehilbert]

Thx for the quick action.