Review: Gate exception field serialization behind langversion 11

This is a well-structured PR that properly gates the behavioral change from #19342 behind LanguageFeature.ExceptionFieldSerializationSupport (F# 11). The approach is sound — emitting GetObjectData and the field-restoring deserialization ctor changes the binary contract of exception types, so gating it is the right call for backward compatibility.

What's good

• Clean gating logic: The emitFieldSerialization boolean at line 12320-12323 is easy to follow and correctly combines langversion check, FSharp.Core exclusion, and empty-fields short-circuit.
• Test coverage: The runtime roundtrip tests and the MessageFieldCollision regression test explicitly opt into langversion 11. The FSharp.Core negative test (Issue_878_FSharpCoreExceptions_NoGetObjectDataOverride) correctly validates that FSharp.Core exceptions are still excluded.
• Baseline IL updates: All baselines that compile at default langversion correctly no longer contain GetObjectData / field-restore IL — this proves the gate works for the default path.
• Release notes: Properly updated to mention the langversion gate.
• Code structure improvement: Moving ilInstrsToSaveFields, ilInstrsForGetObjectData, and ilGetObjectDataDef inside the if emitFieldSerialization then branch avoids constructing those values when they'll never be used.

Minor observations (non-blocking)

1. ilInstrsToRestoreFields still computed unconditionally (lines 12286-12318): Unlike the save/GetObjectData side which was moved inside the if, the restore instructions are computed even when emitFieldSerialization = false. The list is just discarded via the if ... then ilInstrsToRestoreFields else [] at line 12332. Consider moving it inside the branch for symmetry and to avoid the allocation in the common (langversion < 11) path. Not a correctness issue.

2. No explicit negative test: The IL baselines implicitly prove that default langversion doesn't emit field serialization, but an explicit [<Fact>] that compiles an exception at langversion 10 and asserts absence of GetObjectData would be stronger documentation of intent. The existing coverage via Nullness/SerializableAttribute baselines is adequate though.

Overall: clean, minimal, correct.