check-before-deployment: Add info about chronyd/chronyc on CentOS 8 (#5283)

[ti-chi-bot]

This is an automated cherry-pick of #5283

On CentOS 8 Chrony is used by default for NTP services.

• https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_basic_system_settings/using-chrony-to-configure-ntp#sect-checking_if_chrony_is_synchronized

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[TomShawn]

/merge

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This pull request has been accepted and is ready to merge.

Commit hash: e2a37e0