What does this PR do?
Phase 1 of the CI migration:

• remove the CI module, corresponding specs and sigs
• add datadog-ci to the gemspec as runtime dependency
• remove cucumber appraisals
• remove minitest and rspec deps from the contrib appraisal
• update lockfiles
• add datadog-ci to lib-injection scripts

Motivation:
Remove code duplication between gems

Additional Notes:
Not that it is a new runtime dependency I changed host_inject.rb script to install datadog-ci here:
https://github.com/DataDog/dd-trace-rb/pull/3128/files#diff-112f50906c11b7190b1561f372bd7298abe32cda4be0054ced470a584cffb7d3R66

Is there a way to test this? cc @TonyCTHsu

How to test the change?

git clone https://github.com/DataDog/test-environment

cd test-environment

yarn

cd dd-trace-rb/jekyll
bundle add ddtrace --git "https://github.com/DataDog/dd-trace-rb.git" --branch "anmarchenko/migrate_ci_phase_1" --skip-install
bundle install --path .bundle
cd ../..

yarn test dd-trace-rb/jekyll.test.js

cd dd-trace-rb/devdocs
bundle add ddtrace --git "https://github.com/DataDog/dd-trace-rb.git" --branch "anmarchenko/migrate_ci_phase_1" --skip-install
bundle install --path .bundle
cd ../..

yarn test dd-trace-rb/devdocs.test.js

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!