Replace obsolete File.exists? with File.exist?

File.exists has been removed as of Ruby 3.2:
https://www.ruby-lang.org/en/news/2022/12/25/ruby-3-2-0-released/

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

lib/fluent/plugin/opensearch_index_template.rb

@@ -29,15 +29,15 @@
 
 module Fluent::OpenSearchIndexTemplate
   def get_template(template_file)
-    if !File.exists?(template_file)
+    if !File.exist?(template_file)
       raise "If you specify a template_name you must specify a valid template file (checked '#{template_file}')!"
     end
     file_contents = IO.read(template_file).gsub(/\n/,'')
     JSON.parse(file_contents)
   end
 
   def get_custom_template(template_file, customize_template)
-    if !File.exists?(template_file)
+    if !File.exist?(template_file)
       raise "If you specify a template_name you must specify a valid template file (checked '#{template_file}')!"
     end
     file_contents = IO.read(template_file).gsub(/\n/,'')