Drop redundant `require 'set'`

It's no longer needed since Ruby 3.2+ and we dropped Ruby 3.1 some time
ago.

vendor/gems/bundler-checksum/lib/bundler_checksum/command/lint.rb

 # frozen_string_literal: true
 
-require 'set' # rubocop:disable Lint/RedundantRequireStatement -- Ruby 3.1 and earlier needs this. Drop this line after Ruby 3.2+ is only supported.
-
 module BundlerChecksum::Command
   module Lint
     extend self