Use exist? instead of deprecated exists? (#3250)

`Dir.exists?` and `File.exists? were removed by ruby/ruby#5352.
It will be the following error in Ruby 3.2.

```console
% ruby -ve 'File.exists?("foo.rb")'
ruby 3.2.0dev (2021-12-28T15:22:18Z master 39b3aa4fb3) [x86_64-darwin19]
-e:1:in `<main>': undefined method `exists?' for File:Class (NoMethodError)

File.exists?("foo.rb")
    ^^^^^^^^
Did you mean?  exist?
```

And this PR enables `Lint/DeprecatedClassMethods` cop to detect these deprecated methods.

.rubocop.yml

@@ -109,6 +109,9 @@ Layout/TrailingWhitespace:
 Style/RedundantPercentQ:
   Enabled: true
 
+Lint/DeprecatedClassMethods:
+  Enabled: true
+
 # Align `end` with the matching keyword or starting expression except for
 # assignments, where it should be aligned with the LHS.
 Layout/EndAlignment:

lib/install/template.rb

@@ -13,7 +13,7 @@
 say "Copying .browserslistrc to app root directory"
 copy_file "#{__dir__}/config/.browserslistrc", ".browserslistrc"
 
-if Dir.exists?(Webpacker.config.source_path)
+if Dir.exist?(Webpacker.config.source_path)
   say "The JavaScript app source directory already exists"
 else
   say "Creating JavaScript app source directory"
@@ -22,7 +22,7 @@
 
 apply "#{__dir__}/binstubs.rb"
 
-if File.exists?(".gitignore")
+if File.exist?(".gitignore")
   append_to_file ".gitignore" do
     "\n"                   +
     "/public/packs\n"      +