[@azure/eventgrid] Fix Timestamp deserialization (#30399)
### Packages impacted by this PR
@azure/eventgrid
### Issues associated with this PR
NA
### Describe the problem that is addressed by this PR
Fix Timestamp deserialization
### What are the possible designs available to address the problem? If
there are more than one possible design, why was the one in this PR
chosen?
No special design considerations
### Are there test cases added in this PR? _(If not, why?)_
No
### Provide a list of related PRs _(if any)_
https://github.com/Azure/azure-sdk-for-net/pull/44982
### Command used to generate this PR:**_(Applicable only to SDK release
request PRs)_
```autorest --typescript swagger\README.md```
### Checklists
- [X] Added impacted package name to the issue description
- [ ] Does this PR needs any fixes in the SDK Generator?** _(If so, create an Issue in the [Autorest/typescript](https://github.com/Azure/autorest.typescript) repository and link it here)_
- [X] Added a changelog (if necessary)
