fix: use System.nanoTime() for cron check-in duration measurement (#5611)

* fix: use System.nanoTime() for cron check-in duration measurement

System.currentTimeMillis() is a wall-clock value and is subject to
NTP adjustments and DST transitions. For long-running cron jobs this
can produce incorrect or even negative durations in the check-in payload.

Switch the start/end capture in CheckInUtils.withCheckIn() and the three
SentryCheckInAdvice implementations (sentry-spring, sentry-spring-jakarta,
sentry-spring-7) to System.nanoTime(), which is guaranteed monotonic.
Use DateUtils.nanosToSeconds() (already present) to convert the delta.

Fixes #5579

* changelog

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Co-authored-by: Roman Zavarnitsyn <rom4ek93@gmail.com>