SUMMARY
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In the current version of KDE, there is no simple toggle to enable the 24 hour clock globally. There is a way, but it is hidden away behind the undescriptive name "Default (C)". This setting is found in Settings>Regional Settings>Formats, while a person would reasonably expect to find it instead under the Date and Time section.
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OBSERVED RESULT
Unintuitive process to change the time globally to the 24 hour clock. The option is found in Settings>Regional Settings>Formats under the unclear name "Default (C)". Note: this option wasn't even the default, the default option was set to "No change".

EXPECTED RESULT
There would be a simple toggle to enable 24 hour clock under Settings>Regional Settings>Date & Time> Date and Time.

SOFTWARE/OS VERSIONS 
Linux: 5.15.7
KDE Plasma Version: 5.23.4
KDE Frameworks Version: 5.88.0
Qt Version: 5.15.2