Bump puma from 3.12.6 to 4.3.8

Bumps [puma](https://github.com/puma/puma) from 3.12.6 to 4.3.8.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v3.12.6...v4.3.8)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Gemfile

@@ -10,7 +10,7 @@ gem 'rails', '~> 6.0.1'
 # Use postgresql as the database for Active Record
 gem 'pg', '>= 0.18', '< 2.0'
 # Use Puma as the app server
-gem 'puma', '~> 3.12'
+gem 'puma', '~> 4.3'
 # Use SCSS for stylesheets
 gem 'sass-rails', '~> 5.0'
 # Use Uglifier as compressor for JavaScript assets

Gemfile.lock

@@ -117,7 +117,8 @@ GEM
       ast (~> 2.4.1)
     pg (1.2.3)
     public_suffix (4.0.6)
-    puma (3.12.6)
+    puma (4.3.8)
+      nio4r (~> 2.0)
     racc (1.5.2)
     rack (2.2.3)
     rack-proxy (0.6.5)
@@ -260,7 +261,7 @@ DEPENDENCIES
   mysql2 (~> 0.5.2)
   next_rails
   pg (>= 0.18, < 2.0)
-  puma (~> 3.12)
+  puma (~> 4.3)
   rails (~> 6.0.1)
   rspec-rails
   rspec_junit_formatter