Upgrades all dependencies

[dmikusa]

Upgrades all of the dependencies in the Gemfile as well as make the code compliant across Ruby 2.5.9, 2.7.6, and 3.0.4.

Part of this was fixing Rubocop violations & migrating code for Ruby 3.0. Here's the list of what was changed:

• Disable empty blocks under spec tests
• Disable string concat style check
• Change lint name to MissingSuper
• Change ENV[..] usage to ENV.fetch(..., nil)
• Set max for RSpec/MultipleMemoizedHelpers to 10, some tests have a lot of let(..) blocks
• Set RSpec/VerifiedDoubleReference to enforce string style
• Consistently use Naming/VariableNumber, some outliers were snake_case and switched to normalcase
• Disable Lint/UselessMethodDefinition in a few specs where it's necessary
• Remove some unnecessary whitespace, char groups and escaping in regex blocks
• Use be_nil instead of be nil
• Disable Style/GlobalStdStream checks for spec helper
• Reduce complexity of versions_tasks and validate
• Disable helper limit MultipleMemoizedHelpers for droplet helper
• Make the order explicit in a few places by adding parends
• Switch from ifelse to case
• Use File.write
• Remove empty blocks
• Fixes all string alignment issues
• Fixes all string too long issues
• Ignore Ruby 2.6 lints about array slicing since we still need to support Ruby 2.5
• Use an explicit kwargs to be compatible with deprecations and removal in Ruby 2.7 and 3.0
• Bump RubyMine tests to reflect supported version 2.5, 2.7 and 3.0
• Include missing require in filtering_filename_spec.rb that caused it to fail in RubyMine
• Freeze const made from dynamics
• Adjust kwarg expansion so it's compliant for Ruby 3.0 (this requires some version specific behavior in the spec tests)