Update dotenv to v1.0.0 to avoid obsolete method errors

[nikukyugamer]

On Ruby v3.2 over, File.exists? has been removed.
ruby/ruby#5352

And dotenv less than v1.0.0 is used File.exists? so if using Ruby v3.2 later occurs the error as below.

$ RBENV_VERSION=3.2.2 bin/rails s
=> Booting Puma
=> Rails 6.1.7.3 application starting in development 
=> Run `bin/rails server --help` for more startup options
Exiting
/home/takiya/.ghq/github.com/nikukyugamer/auth0-rubyonrails-sample/sample/vendor/bundle/ruby/3.2.0/gems/dotenv-0.11.1/lib/dotenv.rb:6:in `block in load': undefined method `exists?' for File:Class (NoMethodError)

    with(*filenames) { |f| Environment.new(f).apply if File.exists?(f) }
                                                           ^^^^^^^^
Did you mean?  exist?

This deprecated method was removed on v1.0.0.
bkeepers/dotenv@85ad8c5

This PR fixed the above problem.

Note: The latest dotenv version is v2.8.1 but I adopted minimum safer.