[jpa] Do not log failure to persist item with duplicate timestamp as error (#15978)

[jpa] Do not log failure to persist item with duplicate timestamp as error (#15978)

* [jpa] ignore EntityExistsException

in case the user manually added a UNIQUE constraint to the database,
openHAB might send duplicate timestamps.

effectively this means the first attempt is kept, while others are
dropped. as long as you're using sub-second timestamps, this shouldn't
be an issue - the state updates truly should be duplicates

Signed-off-by: Cody Cutrer <cody@cutrer.us>