Observability Edot Dotnet Migrate

It strips out all the old Elastic.Apm.* packages, removes the UseAllElasticApm calls, then wires in Elastic.OpenTelemetry with the correct builder registration. The key gotcha it handles: making sure you set the three required OTEL environment variables and point to a proper OTLP endpoint, not the old APM Server URL. It's narrow in scope but saves you from the documentation hunt when you need to do this exact swap. Works best when you have an existing instrumented app that needs the upgrade.