feat(datetime-api): add TimeZoneDemo to explore java.util.TimeZone with GregorianCalendar

What
- Introduced `TimeZoneDemo` class demonstrating integration of `GregorianCalendar` with `TimeZone`.
- Showcases:
  - Retrieving the system’s default time zone via `gc.getTimeZone()`.
  - Printing display name and ID of the active zone.
  - Switching calendar context to a different zone (`America/Los_Angeles`).
  - Verifying changes by fetching the updated zone ID.

Why
- Time zone handling is critical for globalized applications where scheduling, logging, or transactions span multiple regions.
- Provides a clear, hands-on example of manipulating `TimeZone` objects with legacy `Calendar` APIs.
- Reinforces the difference between default system zones and programmatically set zones.

How to use
- Run `TimeZoneDemo` to view:
  1. Default system time zone.
  2. Human-readable display name.
  3. Time zone ID string.
  4. Updated ID after switching to Los Angeles.
- Modify the `TimeZone.getTimeZone("...")` argument to test with other region identifiers.

Real-life applications
- Converting timestamps between client and server time zones.
- Handling daylight savings adjustments.
- Supporting global applications (meetings, reservations, financial settlements).
- Useful when migrating legacy code using `Calendar`/`TimeZone` before moving to modern `java.time.ZonedDateTime`.

Notes
- `java.util.TimeZone` and `GregorianCalendar` are legacy but still widely used in older systems.
- Prefer modern `java.time` (`ZoneId`, `ZonedDateTime`) for new projects for immutability and thread-safety.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section26DateandTimeAPI/src/TimeZoneDemo.java renamed to Section26DateandTimeAPI/Time/src/TimeZoneDemo.java

@@ -14,7 +14,5 @@ public static void main(String[] args) {
         gc.setTimeZone(TimeZone.getTimeZone("America/Los_Angeles"));
         TimeZone tz1 = gc.getTimeZone();
         System.out.println(tz1.getID());
-
-
     }
 }