refactor(fastapi): migrate HTTP_ROUTE from SpanAttributes to new semantic conventions

Replaced usage of `SpanAttributes.HTTP_ROUTE` with `opentelemetry.semconv.attributes.http_attributes.HTTP_ROUTE`
as part of the migration away from `SpanAttributes`.

Refs: open-telemetry#3475

Author: TheShubhendra

instrumentation/opentelemetry-instrumentation-fastapi/src/opentelemetry/instrumentation/fastapi/__init__.py

@@ -204,7 +204,7 @@ def client_response_hook(span: Span, scope: dict[str, Any], message: dict[str, A
 from opentelemetry.instrumentation.fastapi.version import __version__
 from opentelemetry.instrumentation.instrumentor import BaseInstrumentor
 from opentelemetry.metrics import get_meter
-from opentelemetry.semconv.trace import SpanAttributes
+from opentelemetry.semconv.attributes.http_attributes import HTTP_ROUTE
 from opentelemetry.trace import get_tracer
 from opentelemetry.util.http import (
     get_excluded_urls,
@@ -450,7 +450,7 @@ def _get_default_span_details(scope):
     if method == "_OTHER":
         method = "HTTP"
     if route:
-        attributes[SpanAttributes.HTTP_ROUTE] = route
+        attributes[HTTP_ROUTE] = route
     if method and route:  # http
         span_name = f"{method} {route}"
     elif route:  # websocket