feat(puma): enable setting min puma threads in addition to max (#21048)

config/puma.rb

 persistent_timeout ENV.fetch('PERSISTENT_TIMEOUT') { 20 }.to_i
 
-threads_count = ENV.fetch('MAX_THREADS') { 5 }.to_i
-threads threads_count, threads_count
+max_threads_count = ENV.fetch('MAX_THREADS') { 5 }.to_i
+min_threads_count = ENV.fetch('MIN_THREADS') { max_threads_count }.to_i
+threads min_threads_count, max_threads_count
 
 if ENV['SOCKET']
   bind "unix://#{ENV['SOCKET']}"
@@ -10,7 +11,7 @@ else
 end
 
 environment ENV.fetch('RAILS_ENV') { 'development' }
-workers     ENV.fetch('WEB_CONCURRENCY') { 2 }
+workers     ENV.fetch('WEB_CONCURRENCY') { 2 }.to_i
 
 preload_app!