Document project.json pack removal (#3452)

Author: nkolev92

docs/reference/errors-and-warnings/NU5015.md

@@ -11,14 +11,16 @@ f1_keywords:
 ---
 
 # NuGet Error NU5015
+
+> [!WARNING]
+> project.json support has been removed starting with [NuGet 7.0](../../release-notes/NuGet-7.0.md) and this error will no longer be raised.
+
 > project.json cannot contain multiple Target Frameworks.
 
 ### Issue
 
 The project.json file passed to NuGet pack operation contains multiple target frameworks.
 
-
 ### Solution
 
 Please fix the project.json file to contain only a single target framework.
-

docs/reference/errors-and-warnings/NU5042.md

@@ -9,7 +9,10 @@ f1_keywords:
   - "NU5042"
 ---
 
-# NuGet Error NU5042
+# NuGet Error NU5042 (Obsolete)
+
+> [!WARNING]
+> project.json support has been removed starting with [NuGet 7.0](../../release-notes/NuGet-7.0.md) and this error will no longer be raised.
 
 > `project.json` pack is disabled in the current NuGet version, and will be permanently removed in a future version.
 > Please consider migrating 'MyProject' to `PackageReference` and using the pack targets.
@@ -21,4 +24,4 @@ NuGet.exe pack for project.json projects is not supported.
 
 ### Solution
 
-Migrate your project to PackageReference and use the pack targets instead.
+[Migrate your project to PackageReference](../../archive/project-json.md#migrate-projectjson-to-packagereference) and use the pack targets instead.