Drop Ruby < 2.3

[pirj]

Better viewed ignoring indentation changes https://github.com/rspec/rspec-support/pull/435/files?diff=split&w=1

As per https://github.com/rspec/rspec/issues/61#issuecomment-687290045

Minimal changes to rspec-core and rspec-mocks were required to run rspec-support's own spec suite, pushed to branches with the same name.

What has been done

Done:

• [x] reduce Travis matrix to rubies with complete kwargs support (and compatibility issues with modern openssl)
• [x] reduce Appveyor matrix
• [x] clean up RubyFeatures
• [x] check respond_to?
• [x] check method_defined?
• [x] check send (where we call private methods that later became public)
• [x] cleanup Gemfile
• [x] update required_ruby_version
• [x] update VERSION
• [x] check RUBY_VERSION
• [x] check if we require_rspec_support ruby_features where RubyFeatures are not used
• [-] check define_method where def would suffice
• [+] check (class << self; self; end) usages
• [x] check if Java::JavaLang::String.instance_method(:char_at).arity == -1
• [?] check Java::JavaLang::String.instance_method(:char_at).parameters == [] - still the case
• [x] check 1.8, 1.9, 2.0, 2.1, jruby, 9.0, 9.1, mri
• [x] change old ruby induced rubocop settings, fix offences
• [x] check requires for support's mutex

Discuss:

• [?] introduce https://github.com/ruby/ruby2_keywords dependency and remove respond_to?(:ruby2_keywords, true) check
• [?] cleanup of build functions/predicates
• [?] drop Ruby 2.3 - problematic'ish due to a dependency on outdated OpenSSL, EOL'ed, not supported by RuboCop, still used by 8% of users

TODO:

• [ ] check "work around", "workaround" - there are two cases: check if this was fixed in JRuby 9.2
• [ ] check docs

for rspec-dev:

• [ ] remove unused predicate_functions
• [ ] remove legacy .rubocop settings

[pirj]

I have no idea what's up with Travis, the build shows as "queued", but I don't see any other builds running, including Core and Mocks' rc4.0 branches.

This is worrying:

[JonRowe]

Any PRs working up proposals for RSpec 4.0 need to be marked as draft as they will require very careful release management, they will not be merged to main for a long time.

[JonRowe]

I've created a 4-0-dev branch on all the repos, please target any PRs to that branch

[pirj]

@JonRowe Sounds good, thanks!

May I kindly ask you to grant me rspec/rspec repo branch push permission? I can push to any other repo's branch, but not rspec/rspec. Not being able to push a branch with matching name results in build errors:

Fetching https://github.com/rspec/rspec.git

fatal: Needed a single revision

Git error: command `git rev-parse --verify rc4.0` in directory

[pirj]

I'll close this PR and will open another one with a more descriptive branch name (drop-old-rubies or alike).

[JonRowe]

@pirj just make sure to target 4-0-dev, I'll get it setup properly at the weekend. RSpec 3.x will continue to support the current Ruby matrix

[pirj]

Superseded by #436