## What changes were proposed in this pull request?
Removed unnecessary conversion of microseconds in `DateTimeUtils.timestampToString` to `java.sql.Timestamp` which aims to output fraction of seconds by casting it to string. This was replaced by special `TimestampFormatter` which appends the fraction formatter to `DateTimeFormatterBuilder`: `appendFraction(ChronoField.NANO_OF_SECOND, 0, 9, true)`. The former one means trailing zeros in second's fraction should be truncated while formatting.
## How was this patch tested?
By existing test suites like `CastSuite`, `DateTimeUtilsSuite`, `JDBCSuite`, and by new test in `TimestampFormatterSuite`.
Closes#23936 from MaxGekk/timestamp-to-string.
Lead-authored-by: Maxim Gekk <max.gekk@gmail.com>
Co-authored-by: Maxim Gekk <maxim.gekk@databricks.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>
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