Feature Request: Remove deprecated OpenTracing tracing backends in v25

[timvaillancourt]

Feature Description

In v24, #19619 added a new opentelemetry tracing backend and deprecated the opentracing-jaeger and opentracing-datadog backends. Both log a deprecation warning at startup directing users to migrate to --tracer opentelemetry

Now that v25 development is underway, we should complete the removal:

1. Delete opentracing.go, plugin_jaeger.go, plugin_datadog.go, logger.go from go/trace/
2. Migrate vtadmin/grpcserver from direct opentracing.GlobalTracer() usage to trace.AddGrpcServerOptions()
3. Drop opentracing-go, opentracing-contrib/go-grpc, uber/jaeger-client-go, and dd-trace-go.v1 from go.mod
4. Remove the associated flags: --jaeger-agent-host, --tracing-sampling-type, --datadog-agent-host, --datadog-agent-port, --datadog-trace-debug-mode

This was explicitly planned as v25 cleanup in the #19619 PR description

Use Case(s)

• Reduces the dependency footprint — dd-trace-go.v1 alone pulls in a significant number of transitive deps (DataDog agent packages, etc.)
• Simplifies the tracing code to a single implementation path (OpenTelemetry)
• The OpenTracing project itself is archived and recommends migrating to OpenTelemetry