Reduce production gem size

Include only files that are needed for the production gem.

Also avoids to use git to allow to build the package in an environment
that does not have git (on purpose).

### Before

173K

```
.dokaz
.github/PULL_REQUEST_TEMPLATE.md
.github/dependabot.yml
.github/workflows/dependency-review.yml
.github/workflows/main.yml
.gitignore
.rspec
.rubocop.yml
.ruby-version
.yardopts
CHANGELOG.md
CODE_OF_CONDUCT.md
Gemfile
Gemfile.lock
LICENSE.txt
README.md
Rakefile
benchmarks/Gemfile
benchmarks/benchmarks.rb
bin/console
bin/setup
lib/memo_wise.rb
lib/memo_wise/internal_api.rb
lib/memo_wise/version.rb
logo/logo.png
memo_wise.gemspec
```

### After

19K

```
CHANGELOG.md
LICENSE.txt
README.md
lib/memo_wise.rb
lib/memo_wise/internal_api.rb
lib/memo_wise/version.rb
```

Ref: https://docs.rubocop.org/rubocop-packaging/cops_packaging.html#packaginggemspecgit

CHANGELOG.md

@@ -9,6 +9,8 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 **Gem enhancements:**
 
+- Reduced production gem size by 9 times [[#345](https://github.com/panorama-ed/memo_wise/pull/345)]
+
 _No breaking changes!_
 
 **Project enhancements:**

memo_wise.gemspec

@@ -26,13 +26,7 @@ Gem::Specification.new do |spec|
   spec.required_ruby_version = Gem::Requirement.new(">= 2.5.0")
 
   # Specify which files should be added to the gem when it is released.
-  # The `git ls-files -z` loads the files in the RubyGem that have been added
-  # into git.
-  spec.files = Dir.chdir(File.expand_path(__dir__)) do
-    `git ls-files -z`.split("\x0").reject do |f|
-      f.match(%r{^(test|spec|features)/})
-    end
-  end
+  spec.files = Dir.glob('{CHANGELOG.md,LICENSE.txt,README.md,lib/**/*.rb}')
   spec.require_paths = ["lib"]
 
   spec.metadata = {