Upgrade RuboCop to v1.30

[mishina2228]

RuboCop v1.30.0 can now parse Ruby 2.0 code.
This PR upgrades RuboCop to v1.30 and also fixes and suppresses some cops.
It was hard to fix all cops, so I fixed only the simple ones.

This PR supersedes #1256.

[sodabrew]

Looks great, thanks for doing this! The only specific change I'd like is removing the extra psych gem that was pinned lower for older versions of RuboCop.

The other comment about older versions of Rake can be dealt with in a Ruby 2.x or higher pass. There may be some other old syntax or old C extension methods that can be cleaned up as well.

[sodabrew]

I think the next release could drop Ruby < 2.2

[mishina2228]

I tried to omit if-else condition, but rake 13.x supports only Ruby 2.2+.
So can we drop Ruby 2.1 from the CI matrix?

mysql2/.github/workflows/build.yml

Lines 18 to 27 in 80e0001

	ruby:
	- '3.1'
	- '3.0'
	- '2.7'
	- '2.6'
	- '2.5'
	- '2.4'
	- '2.3'
	- '2.2'
	- '2.1'

and also CentOS 7 with Ruby 2.0.0?

mysql2/.github/workflows/container.yml

Lines 14 to 15 in 80e0001

	# CentOS 7 system Ruby is the fixed version 2.0.0.
	- {distro: centos, image: 'centos:7', name_extra: 'ruby 2.0.0'}