Allow faraday-net_http up to v3.1

Version 3.0 of `faraday-net_http` uses the new streaming API and the new `finished` option in `save_response`, which are not available in `faraday` prior to version 2.5, and has therefore been released as a new major version.

To avoid the same issue in future, we're fixing the version to allow any 3.0.x version, but not 3.1+.
This will allow us to ship a minor versions of the adapter with "breaking" changes for older versions of Faraday.

faraday.gemspec

@@ -15,7 +15,13 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = '>= 2.6'
 
-  spec.add_dependency 'faraday-net_http', '~> 2.0'
+  # faraday-net_http is the "default adapter", but being a Faraday dependency it can't
+  # control which version of faraday it will be pulled from.
+  # To avoid releasing a major version every time there's a new Faraday API, we should
+  # always fix its required version to the next MINOR version.
+  # This way, we can release minor versions of the adapter with "breaking" changes for older versions of Faraday
+  # and then bump the version requirement on the next compatible version of faraday.
+  spec.add_dependency 'faraday-net_http', '>= 2.0', '< 3.1'
   spec.add_dependency 'ruby2_keywords', '>= 0.0.4'
 
   # Includes `examples` and `spec` to allow external adapter gems to run Faraday unit and integration tests