[Fix #6675] Avoid printing deprecation warnings about constants

[elmasantos]

This PR prevents deprecation warnings about constants to be shown (issue #6675).

It was needed to implement a method called silence_warnings, since Kernel::silence_warnings was hiding RuboCop's warnings.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Thanks for tackling this! The changes look good to me. Just add an entry in the changelog regarding them.

[elmasantos]

Thanks for the reply @bbatsov! I added an entry in the changelog. Could you check again? :D

[bbatsov]

Thanks!