- adds upgrade guides for v4 and v5

Author: baywet

docs/upgrade-to-v4.md

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+# Microsoft Graph Java SDK v4 changelog and upgrade guide
+
+Welcome to the Microsoft Graph Java SDK migration guide to v4. The purpose of this document is to list out any breaking change and migration work SDK users might run into while upgrading to the latest version of the SDK.
+
+## Upgrade guide for breaking changes
+
+### IdentitySet replaced by specialized types for Microsoft Teams APIs
+
+The `IdentitySet` type has been replaced by specialized types inheriting from it and exposing more information for the Microsoft Teams APIs.
+
+- The `ChatMessage.from` property is now of type `ChatMessageFromIdentitySet`
+- The `ChatMessageMention.mentioned` property is now of type `ChatMessageMentionedIdentitySet`
+- The `ChatMessageReaction.user` property is now of type `ChatMessageReactionIdentitySet`
+
+If your application is reading/assigning the above-mentioned properties, you need to update the code to reference these new types instead. If your application was using the `additionalData` property to read any property that is now available on the new types, update the code to use these new properties instead.

docs/upgrade-to-v5.md

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+# Microsoft Graph Java SDK v5 changelog and upgrade guide
+
+Welcome to the Microsoft Graph Java SDK migration guide to v5. The purpose of this document is to list out any breaking change and migration work SDK users might run into while upgrading to the latest version of the SDK.
+
+## Upgrade guide for breaking changes
+
+### ManagedAppProtection replaced by TargetedManagedAppProtection for some Security APIs
+
+The `ManagedAppProtection` type used for the `targetApps` action as a parameter has been replaced by a new `TargetedManagedAppProtection` type that inherits from it and exposes new properties.
+
+If your application is using the above-mentioned type, you need to update the code to reference the new type instead. If your application was using the `additionalData` property to read any property that is now available on the new type, update the code to use these new properties instead.