Add 15.4 breaking changes for NTP host

What does this MR do?

Document actions required for 15.4 air-gapped environment.

Related issues

This is a TO-DO item from emergency ticket. It is important to add this to the breaking changes note so that customers running in an air-gapped environment will not be caught by this issue.

More details in another MR: Add ntp_host config for gitaly cluster docs (!100629 - merged)

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