test: add appraisal for an unlocked/nil version (open-telemetry#1636)

1. This will test instrumentation against the latest available HTTPClient gem
2. Closes open-telemetry#1600

Author: arjun-rajappa

instrumentation/http_client/Appraisals

@@ -12,6 +12,10 @@
 semconv_stability = %w[dup stable old]
 
 semconv_stability.each do |mode|
+  appraise "httpclient-latest-#{mode}" do
+    gem 'httpclient'
+  end
+
   appraise "httpclient-2.8-#{mode}" do
     gem 'httpclient', '~> 2.8.0'
   end