fix: pin rubocop to 1.12 due to error with ruby 2.4

Rubocop 1.13 was released today (2021-04-20) and it has dropped support
for Ruby 2.4[1][2].  This causes rubocop to die on the spot when
trying to run it:

    $ bundle exec rubocop --version
    1.13.0
    $ bundle exec rubocop
    Error: RuboCop found unsupported Ruby version 2.4 in `TargetRubyVersion` parameter (in .rubocop.yml). 2.4-compatible analysis was dropped after version 1.12.
    Supported versions: 2.5, 2.6, 2.7, 3.0

And thus the "style_check" ci job is currently broken[3].

Rubocop 1.10 and 1.11 don't work either because of a rename:

    $ bundle exec rubocop
    Error: unrecognized cop Style/StringChars found in .rubocop.yml
    Did you mean `Style/StringMethods`?

Also, for some reason bundler installs rubocop 1.13 even with the
current "~> 1.10" specification.  This happens both on my machine (Artix
Linux; ruby 3.0.1p64; rubygems 3.2.15; bundler 2.2.16) and on GitHub
Actions (Ubuntu 20.04; ruby 2.5; rubygems ?; bundler 2.2.16)[3].

So pin rubocop to the 1.12 series using the usual >= and < operators.

Example of another project doing something similar: [4].

Fixes jekyll#8649.

[1] https://github.com/rubocop/rubocop/releases/tag/v1.13.0
[2] rubocop/rubocop#9648
[3] https://github.com/jekyll/jekyll/runs/2394239794?check_suite_focus=true
[4] Shopify/job-iteration#79

Gemfile

@@ -23,7 +23,7 @@ group :test do
   gem "nokogiri", "~> 1.7"
   gem "rspec"
   gem "rspec-mocks"
-  gem "rubocop", "~> 1.10"
+  gem "rubocop", ">= 1.12", "< 1.13"
   gem "rubocop-minitest"
   gem "rubocop-performance"
   gem "rubocop-rake"