.Net: Mark 'Content' Property of 'ChatMessageContent' class obsolete

[SergeyMenshykh]

Today, the 'Content' property of the 'ChatMessageContent' class serves as a shortcut for the first item of the TextContent type. This can be confusing for consumer code, in cases where a message contains multiple items of the TextContent type.

The proposal here is to mark the 'Content' property as obsolete and eventually removing it after providing consumers with sufficient time to migrate to using the Items collection.

More details - #5088 (comment)

[github-actions]

This issue is stale because it has been open for 90 days with no activity.

[SergeyMenshykh]

To stay aligned with our commitment to no breaking changes, we are going to keep the property.