Disable heap profiling spec on legacy rubies

Oooops! This spec definitely should not be running on legacy Rubies.

It actually started to flake with a recent change, and it took me
a while to actually remember that it shouldn't even be running in
the first place.

spec/datadog/profiling/collectors/cpu_and_wall_time_worker_spec.rb

@@ -654,6 +654,7 @@
       let(:options) { { dynamic_sampling_rate_enabled: false } }
 
       before do
+        skip 'Heap profiling is only supported on Ruby >= 2.7' if RUBY_VERSION < '2.7'
         allow(Datadog.logger).to receive(:warn)
         expect(Datadog.logger).to receive(:warn).with(/dynamic sampling rate disabled/)
       end