fix: use ActiveSupport from top-level namespace

[ryanharkins]

Description

Came across an error when ActiveRecord Instrumentation is being installed in a Rails app.

Proposed change

Use ::ActiveSupport to look for constant in the top-level namespace.

Error:

OpenTelemetry error: Instrumentation: OpenTelemetry::Instrumentation::ActiveRecord unhandled exception during install:
...
undefined method `on_load' for OpenTelemetry::Instrumentation::ActiveSupport:Module
...
ActiveSupport.on_load(:active_record) do
             ^^^^^^^^

[linux-foundation-easycla]

• ✅login: ryanharkins / (e966580)

The committers listed above are authorized under a signed CLA.