Fix kwargs deprecation warnings for Ruby 2.7.2

This MR upgrades to Ruby 2.7.2, enables deprecation warnings for specs, ignores .bundle directory, and fixes kwargs deprecation warnings.

Please see individual commit messages for more details.

Contributes to gitlab-org/gitlab#257438 (closed).

Screenshots

Before	After

The remaining deprecation warning

/home/peter/.dotfiles/asdf/installs/ruby/2.7.2/lib/ruby/gems/2.7.0/gems/rspec-mocks-3.10.0/lib/rspec/mocks/message_expectation.rb:101: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/home/peter/devel/gitlab/labkit-ruby/lib/labkit/tracing/tracing_utils.rb:11: warning: The called method `with_tracing' is defined here

stems from https://github.com/rspec/rspec-mocks/issues/1306.