chore: drop unused dev dependencies

Remove four dev dependencies that are not referenced anywhere in the gem,
its specs, the Rakefile, the Guardfile, the bin/ scripts, the examples, or
the CI workflow:

- `codecov` (~> 0.1): Officially deprecated by Codecov in 2021 in favour
  of the Codecov uploader binary / GitHub Action. The current
  `.github/workflows/tests.yml` uploads the simplecov report as a GitHub
  artifact and never invokes the codecov gem.
- `rspec-legacy_formatters` (~> 1.0): The Gemfile comment states it was
  only there for the codecov formatter. Drops with codecov.
- `rspec_junit_formatter` (~> 0.4): The CI workflow runs
  `bundle exec rspec --format documentation` and produces no JUnit XML,
  so this formatter has no consumer.
- `erb`: The standalone gem has no consumer in the project. ERB is in the
  Ruby stdlib for the supported matrix (>= 2.7), and `require 'erb'` is
  not used anywhere in the gem.

Verified: full RSpec suite (151 examples, 1 failure - pre-existing
`truncate_len` integration failure on master, unrelated to this change)
and `bundle exec rubocop` (97 files, no offenses) both green after the
removal.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erimicel

Gemfile

@@ -7,14 +7,10 @@ git_source(:github) { |repo_name| "https://github.com/#{repo_name}" }
 # Dev dependencies
 gem 'awesome_print', '~> 1.8'
 gem 'bundler', '~> 2.0'
-gem 'codecov', '~> 0.1'
-gem 'erb'
 gem 'guard', '~> 2.16'
 gem 'guard-rubocop', '~> 1.3'
 gem 'rake', '~> 13.0'
 gem 'rspec', '~> 3.9'
-gem 'rspec_junit_formatter', '~> 0.4'
-gem 'rspec-legacy_formatters', '~> 1.0' # For codecov formatter
 gem 'rubocop', '1.72.2'
 gem 'rubocop-rspec', '~> 3.6', require: false
 gem 'simplecov', '~> 0.18'