Make DistributedContextPropagator opt-in (keep OTel propagation backwards compatible until v11)

[ramonsmits]

Stacks on #7798 (otel). Keeps the new DistributedContextPropagator-based OpenTelemetry context propagation opt-in so 10.3 stays backwards compatible, with the new behavior becoming the default in v11.

Why

otel (commit bba80f7) replaced the hand-written baggage propagator with System.Diagnostics.DistributedContextPropagator. The HTTP header names are unchanged (traceparent/tracestate/baggage were already W3C), but the baggage value encoding changes:

	Legacy (≤10.2)	New (DistributedContextPropagator)
Wire format	key1=value1,key2=value2	key1 = value1, key2 = value2 (W3C OWS)
Value encoding	Uri.EscapeDataString whole value	W3C baggage encoding
Value whitespace	preserved (percent-encoded)	trimmed

ContextPropagationIncompatibilityTests documents the breakage: new→legacy gains a leading space, and new round-trips strip value whitespace. That's breaking on rolling upgrades, so it can't ship default-on in a minor.

What

• Default = legacy (backwards compatible). New propagator is opt-in via AppContext switch NServiceBus.Core.OpenTelemetry.UseDistributedContextPropagator, mirroring the existing AppContextSwitches.UseV2DeterministicGuid / [PreObsolete] pattern (flip default in v11, remove the switch + legacy code in v11).
• All temporary code in obsolete_v11.cs — the switch plumbing plus a byte-for-byte revert of the pre-10.3 propagator. v11 cleanup = delete that one file + remove the two if (!ObsoleteV11.UseDistributedContextPropagator) delegation blocks in ContextPropagation.cs.
• ContextPropagation.cs keeps the new DistributedContextPropagator implementation as the permanent/future path.
• Span naming (UseMessageDestinationInSpanNames via the public Tracing() API) is already opt-in/non-breaking — left untouched.

Enabling the switch:

AppContext.SetSwitch("NServiceBus.Core.OpenTelemetry.UseDistributedContextPropagator", true);
// or <RuntimeHostConfigurationOption Include="NServiceBus.Core.OpenTelemetry.UseDistributedContextPropagator" Value="true" />
// or env (.NET 9+): DOTNET_NServiceBus_Core_OpenTelemetry_UseDistributedContextPropagator=true