Replace Instant with String for lastModified in Annotations (#383)

Replace `Instant` with `String` for `lastModified` in `Annotations`

## Motivation and Context
To fix serialization/kotlin library version conflicts 

## How Has This Been Tested?
Unit test

## Breaking Changes
Yes

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/WebSocketMcpKtorServerExtensions.kt

@@ -71,7 +71,7 @@ internal suspend fun WebSocketServerSession.mcpWebSocketEndpoint(block: () -> Se
     try {
         session = server.createSession(transport)
         awaitCancellation()
-    } catch (e: CancellationException) {
+    } catch (_: CancellationException) {
         session?.close()
     }
 }