Usage of Java's `LocalTime` instead of Kotlinx's

[Jolanrensen]

Convert toLocalTime uses Java's LocalTime instead of Kotlinx's.
We should use kotlinx.datetime.LocalTime instead of java.time.LocalTime if we ever want to go multiplatform and just for general consistency.

parse uses even more java.time versions: LocalDate, LocalDateTime, LocalTime,
as well as java.time.format.DateTimeFormatter, but I'm not sure if that can be replaced.

[devcrocod]

It’s hard for me to say now why java.time is used in parser
I assume that back then kotlinx-datetime was more limited in its capabilities

And so it is necessary to save java.time in the public api, since often use it anyway, but have a similar api for kotlinx-datetime. As for internal things, yea, it would be nice to have kotlinx-datetime