rspec (2.6, *) workflows fail due to Rubocop version not supporting Ruby 2.6

[fulf]

As of #818, Ruby 2.6 test workflows fail with the following error:

Could not find compatible versions

Because rubocop >= 1.51.0 depends on Ruby >= 2.7.0
  and Gemfile depends on rubocop = 1.59.0,
  Ruby >= 2.7.0 is required.
So, because current Ruby version is = 2.6.10,
  version solving has failed.
Error: The process '/opt/hostedtoolcache/Ruby/2.6.10/x64/bin/bundle' failed with exit code 6