Move rake development dependency to Gemfile (#603)

## All PRs:

* [x] Has tests
* [ ] Documentation updated

Move `rake` from `secure_headers.gemspec` into the alphabetized `:test`
group in `Gemfile`. This fixes the `Gemspec/DevelopmentDependencies`
offense enabled by `rubocop-github` 0.29.0, which caused the Ruby 3.4
and 4.0 jobs in [the failing main
build](https://github.com/github/secure_headers/actions/runs/37043211037)
to fail.

The runtime `cgi` dependency is unchanged. No dependency pins, lockfile,
or CI configuration changes are included. Documentation updates and new
tests are not applicable to this development-dependency relocation; the
existing suite and lint checks cover the change.

Validation used `export RBENV_VERSION=4.0.5`, Bundler 4.0.19, RuboCop
1.91.0, and `rubocop-github` 0.29.0:

| Command | Result |
| --- | --- |
| `bundle exec rubocop --only Gemspec/DevelopmentDependencies
secure_headers.gemspec` (before the fix) | Failed as expected with the
reported line-34 offense |
| `bundle exec rubocop` (after the fix) | Passed: 48 files, no offenses
|
| `bundle exec rspec spec` | Passed: 322 examples, 0 failures |
| `bundle exec rake --version` | Passed: rake 13.4.2 remains available
through Bundler |
| `bundle exec rake -T` | Passed: project tasks load successfully |
| `git diff --check` | Passed |

RSpec emitted a nonblocking Coveralls/SimpleCov deprecation warning.
Other Ruby matrix versions were not run locally.

## Adding a new header

Generally, adding a new header is always OK.

* Is the header supported by any user agent? If so, which? N/A; no
header added.
* What does it do? N/A.
* What are the valid values for the header? N/A.
* Where does the specification live? N/A.

## Adding a new CSP directive

* Is the directive supported by any user agent? If so, which? N/A; no
directive added.
* What does it do? N/A.
* What are the valid values for the directive? N/A.

Author: fletchto99

Gemfile

@@ -10,6 +10,7 @@ group :test do
   gem "json"
   gem "pry-nav"
   gem "rack"
+  gem "rake"
   gem "rspec"
   gem "rubocop"
   gem "rubocop-github"

secure_headers.gemspec

@@ -30,6 +30,4 @@ Gem::Specification.new do |gem|
   gem.extra_rdoc_files = Dir["README.md", "CHANGELOG.md", "LICENSE"]
 
   gem.add_dependency "cgi", ">= 0.1"
-
-  gem.add_development_dependency "rake"
 end