fix(stjm): add missing IMigrateFrom interface to PerfStaticV2

PerfStaticV2 had a TryMigrateFrom method but did not declare
IMigrateFrom<PerfStaticV1, PerfStaticV2> in its interface list.
The library discovers static migrators by the interface, not
just the method signature, so the StaticMigration benchmarks
threw 'No migrator was found for discriminator'.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: egil

Egil.SystemTextJson.Migration/perf/Egil.SystemTextJson.Migration.PerfTests/MigrationScenarioBenchmarks.cs

@@ -224,7 +224,7 @@ public static PerfStaticPlainV2 ManualFrom(PerfStaticPlainV1 source)
 public record class PerfStaticV1(string Name, int Age, string[] Tags);
 
 [JsonMigratable]
-public record class PerfStaticV2(string FirstName, string LastName, int Age, string[] Tags) : IJsonMigrationTracked
+public record class PerfStaticV2(string FirstName, string LastName, int Age, string[] Tags) : IJsonMigrationTracked, IMigrateFrom<PerfStaticV1, PerfStaticV2>
 {
     [JsonIgnore]
     public bool MigratedDuringDeserialization { get; set; }