refactor the convert_to_utc_date function

Author: karangattu

tests/playwright/shiny/inputs/input_slider/test_input_slider_app.py

@@ -11,8 +11,7 @@
 
 def convert_to_utc_date(date_str: str) -> str:
     date_obj = datetime.datetime.strptime(date_str, "%m/%d/%y")
-    epoch_time_seconds = date_obj.timestamp()
-    epoch_time_milliseconds = float(epoch_time_seconds * 1000)
+    epoch_time_milliseconds = date_obj.timestamp() * 1000
     return str(epoch_time_milliseconds)