Ensure using single Utc object in TimeZoneInfo (#24614)

Ensure using single Utc object in TimeZoneInfo (#24614)

Currently if calling GetSystemTimeZones, will return a list which in include Utc object created from the enumeration.  But when calling FindSystemTimeZoneById("UTC") will return the cached Utc object which is different than the one returned from the enumeration. This in general could be not a problem but it is better to ensure returned the same object from both APIs. The reason is in our internal code, we use some reference equality for the perf reason. Ensure the usage of Utc object will make it more reliable.