Mmissing leading zeros in DateTime year serialization and timezone offset formatting #254

During testing of DateTime serialization, two issues were identified in the NETJSON serializer.

The first issue is related to the formatting of years with fewer than four digits. This issue has been fixed in .NET 10, but it is still reproducible on .NET Framework 4.7.2.
The second issue is related to the formatting of the local timezone offset.

1. Years below 1000 are not zero-padded
   NETJSON does not always serialize years using the required four-digit format.
   .NET 10: works correctly.
   .NET Framework 4.7.2: leading zeros are missing.
   Expected:
   0026-09-03T15:36:22.9827969+03:00

2. Local timezone offset is not ISO 8601 formatted
   NETJSON serializes the UTC offset without a colon:
   Actual:
   "0026-09-03T15:36:22.9827969+0300"
   Expected:
   "0026-09-03T15:36:22.9827969+03:00"

Both issues result in NETJSON producing DateTime strings that are not consistently compliant with the expected ISO 8601 representation.