chore: ignore obsolete warnings in tests

Author: Tarmil

tests/FSharp.SystemTextJson.Tests/Test.Record.fs

@@ -1,5 +1,7 @@
 module Tests.Record
 
+#nowarn 44 // ignore obsolete on IgnoreNullValues
+
 open Xunit
 open System.Text.Json.Serialization
 open System.Text.Json

tests/FSharp.SystemTextJson.Tests/Test.Regression.fs

@@ -1,5 +1,7 @@
 module Tests.Regression
 
+#nowarn 44 // ignore obsolete on IgnoreNullValues
+
 open Xunit
 open System.Text.Json.Serialization
 open System.Text.Json

tests/FSharp.SystemTextJson.Tests/Test.Union.fs

@@ -1,5 +1,7 @@
 module Tests.Union
 
+#nowarn 44 // ignore obsolete on IgnoreNullValues
+
 open System.Collections.Generic
 open Xunit
 open System.Text.Json.Serialization