Explain cancelled CLI deprecations clearly

[tnir]

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

Cancelled CLI deprecations in 2019 just before the release of Bundler 2.1.0 have been hard to understand and 3 years passed after.

Especially an introduction of bundle config new subcommand-based interface (rubygems/bundler#7561) was not postponed.

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

• Groups two CLI deprecations cancelled in Bundler 3
• Rewrites disallowance of old subcommand interface in bundle config.
• Uses headings 3 for CLI, Helper, and DSL sections instead of headings 4.

Updates rubygems/bundler#7561

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

Resources

• Delay deprecation of bundle config and bundle update without args bundler#7475
• Integrate UPGRADING from the main repo bundler-site#721

Signed-off-by: Takuya Noguchi takninnovationresearch@gmail.com