Deserialization of SBOM defaults the metadata/timestamp

```java
public class Metadata extends ExtensibleElement {

@JsonSerialize(using = CustomDateSerializer.class)
@VersionFilter(versions = {"1.2", "1.3", "1.4"})
private Date timestamp = new Date();
```

This is useful (perhaps) when creating a new SBOM programmatically, to then be serialized. It is misleading when deserializing an existing SBOM without a timestamp where the current date/time may not be expected.