manager: add structured log message about clock bump

manager: add structured log message about clock bump

Requested in https://github.com/systemd/systemd/pull/33214#discussion_r1630251308.
Also, reword error messages a bit. When /usr/lib/clock-epoch was introduced,
"build time" stopped being acurate. Just say "epoch" instead.

The same message ID is used in the manager and timesyncd. The event is
essentially equivalent for the user, and it seems reasonable that to search for
both at the same time.

The catalog entry is dropped. It provided almost no additional information above
the message. When the same message ID is now applied to messages from PID1 and
timesyncd, and the clock can be both advanced and rewound, it becomes very hard
to make the catalog entry provide something useful, because catalog entries don't
allow conditionalization.