Don't scaffold default index method annotation

When scaffolding, we have an Npgsql-specific IndexMethod annotation to
represent PostgreSQL's index methods. Previously, we would always
output this annotation on indices, even when the method was the
default (btree); NpgsqlAnnotationCodeGenerator would elide it as by-
convention.

Although this is cleaner, it caused issues whenever two indices where
defined on the same column:
dotnet/efcore#11846

We now scaffold the annotation only for non-default index methods as a
workaround; the issue should now affect much less people.

Fixes #228

(cherry picked from commit 38d2176)

src/EFCore.PG/Scaffolding/Internal/NpgsqlDatabaseModelFactory.cs

@@ -364,7 +364,14 @@ void GetIndexes()
                         }
                     }
 
-                    index[NpgsqlAnnotationNames.IndexMethod] = reader.GetValueOrDefault<string>("amname");
+                    // It's cleaner to always output the index method on the database model,
+                    // even when it's btree (the default);
+                    // NpgsqlAnnotationCodeGenerator can then omit it as by-convention.
+                    // However, because of https://github.com/aspnet/EntityFrameworkCore/issues/11846 we omit
+                    // the annotation from the model entirely.
+                    var indexMethod = record.GetValueOrDefault<string>("amname");
+                    if (indexMethod != "btree")
+                        index[NpgsqlAnnotationNames.IndexMethod] = indexMethod;
 
                     table.Indexes.Add(index);
                 }