Adopt Ruby Community Conduct Guideline for Code of Conduct

[dysonreturns]

What was the end-user or developer problem that led to this PR?

rubygems and bundler are now maintained by Ruby core.
The current Code of Conduct (Contributor Covenant) for these have been rejected by Ruby, who created the Ruby Community Conduct Guideline to be used instead.
See: https://bugs.ruby-lang.org/issues/12004
Result: https://www.ruby-lang.org/en/conduct/

The Contributor Covenant puts contributors' personal lives under scrutiny and allows users to be punished for actions they take outside of the project. This is governance over-reach. It is also very long and changes with version. :/

It was a decision made by previous maintainers, which we should now re-evaluate.

Problem 1: bundler and rubygems needs Ruby Community Conduct Guideline

These should be unified for same developer experience and community governance which has served Ruby well.

Problem 2. The default Code of Conduct for a new gem is out of sync with Ruby

Currently, the default supplied Code of Conduct (CODE_OF_CONDUCT.md) when creating a gem with bundler puts the Contributor Covenant as a markdown file in your project.

If we fix Problem 1, then Ruby, rubygems, bundler (and even Rails) all use the Ruby Community Conduct Guideline.
Then it would make no sense to distribute the Contributor Covenant when creating a Ruby gem.
We must therefore adopt the Ruby Community Conduct Guideline as default CODE_OF_CONDUCT.md file for a new gem.

What is your fix for the problem, implemented in this PR?

• Change the project Code of Conduct to Ruby Community Conduct Guideline
• Change default Code of Conduct for new gem to Ruby Community Conduct Guideline
• Change new gem prompt when asking for CoC

Please help check wording in the files

I am not a good writer, but I tried my best. Please ask Ruby Core to review this version to see if it's ok for project and distribution:
CODE_OF_CONDUCT.md and copy bundler/lib/bundler/templates/newgem/CODE_OF_CONDUCT.md.tt

Thank you!

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
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