Remove Coveralls gem, update simplcov

Coveralls have changed how they work, the gem is no longer supported,
instead they rely on simplecov, simplecov-lcov and a GitHub action.

Here we revove the Coveralls gem and update simplecov. The coverage is
now output to `coverage/lcov.info`.

Rakefile

@@ -1,12 +1,7 @@
-# frozen_string_literal: true
-
 require "bundler/gem_tasks"
 require "rspec/core/rake_task"
 require "standard/rake"
-require "coveralls/rake/task"
-
-Coveralls::RakeTask.new
 
 RSpec::Core::RakeTask.new(:spec)
 
-task default: %i[standard spec coveralls:push]
+task default: %i[standard spec]

mail-notify.gemspec

@@ -26,7 +26,8 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_development_dependency "bundler", "~> 2.0"
-  spec.add_development_dependency "coveralls", "~> 0.8.22"
+  spec.add_development_dependency "simplecov", "~> 0.21"
+  spec.add_development_dependency "simplecov-lcov"
   spec.add_development_dependency "pry", "~> 0.14.1"
   spec.add_development_dependency "rails", "7.0.8"
   spec.add_development_dependency "rake", "~> 13.0.6"

spec/spec_helper.rb

@@ -1,7 +1,9 @@
 # frozen_string_literal: true
 
-require "coveralls"
-Coveralls.wear!
+require 'simplecov'
+require 'simplecov-lcov'
+SimpleCov.formatter = SimpleCov::Formatter::LcovFormatter
+SimpleCov.start
 
 require "bundler/setup"
 require "action_mailer"