Avoid using problematic rspec-mocks 3.11.2

This version of rspec-mocks breaks CI for Ruby 2.7, 3.0 and 3.1 on
the following tests:

* ./spec/datadog/core/configuration_spec.rb:377 (Ruby 2.7)
* ./spec/datadog/core_spec.rb:35 (Ruby 2.7)
* ./spec/ddtrace/transport/http/builder_spec.rb:250 (Ruby 3.0, 3.1)

References:

* <https://app.circleci.com/pipelines/github/DataDog/dd-trace-rb/7560/workflows/24205733-d9fb-4781-962c-87499bad162d/jobs/280394>
* <https://app.circleci.com/pipelines/github/DataDog/dd-trace-rb/7560/workflows/24205733-d9fb-4781-962c-87499bad162d/jobs/280387>
* <https://app.circleci.com/pipelines/github/DataDog/dd-trace-rb/7560/workflows/24205733-d9fb-4781-962c-87499bad162d/jobs/280396>

I have not yet reported this upstream, but wanted to first unblock our
CI.

Gemfile

@@ -28,8 +28,9 @@ end
 gem 'rake', '>= 10.5'
 gem 'rake-compiler', '~> 1.1', '>= 1.1.1' # To compile native extensions
 gem 'redcarpet', '~> 3.4' if RUBY_PLATFORM != 'java'
-gem 'rspec', '~> 3.10'
+gem 'rspec', '~> 3.11'
 gem 'rspec-collection_matchers', '~> 1.1'
+gem 'rspec-mocks', '!= 3.11.2'
 if RUBY_VERSION >= '2.3.0'
   gem 'rspec_junit_formatter', '>= 0.5.1'
 else