Marked StringCatalog as Windows-only

Also marked its test class with the same. Also marked another test class Windows-only, though it doesn't have to be Windows-only (just how it was written). This attribute is only supported with .NET5 and later, so on Framework there will be no marking.

These changes fix a large number of build warnings.

SIL.Core.Desktop.Tests/i18n/StringCatalogTests.cs

@@ -10,6 +10,10 @@ namespace SIL.Tests.i18n
 {
 
 	[TestFixture]
+#if NET
+	[System.Runtime.Versioning.SupportedOSPlatform("windows")]
+#endif
+	[Platform("windows")]
 	public class StringCatalogTests
 	{
 		private string _poFile = Path.GetTempFileName();

SIL.Core.Desktop/i18n/StringCatalog.cs

@@ -6,6 +6,9 @@
 
 namespace SIL.i18n
 {
+#if NET
+	[System.Runtime.Versioning.SupportedOSPlatform("windows")]
+#endif
 	public class StringCatalog
 	{
 		private Dictionary<string, string> _catalog;

SIL.DictionaryServices.Tests/LexEntryRepositoryCachingTests.cs

@@ -11,7 +11,11 @@
 namespace SIL.DictionaryServices.Tests
 {
 	[TestFixture]
+	[Platform(Include = "Windows")]
 	[OfflineSldr]
+#if NET
+	[System.Runtime.Versioning.SupportedOSPlatform("windows")]
+#endif
 	public class LiftLexEntryRepositoryCachingTests
 	{
 		private TemporaryFolder _tempfolder;
@@ -371,4 +375,4 @@ public void GetAllEntriesSortedByDefinition_DeleteAllItemsAfterFirstCall_EntryIs
 
 
 	}
-}
+}