DEV: Bump puma from 3.12.2 to 4.3.1 (#8475)

The downside here is that we are now going to require nio4r which requires some native code.

It is reasonably low risk though cause puma is very well tested. We use puma mostly in test environments, not in production.

Gemfile.lock

@@ -204,6 +204,7 @@ GEM
     multi_xml (0.6.0)
     multipart-post (2.1.1)
     mustache (1.1.0)
+    nio4r (2.5.2)
     nokogiri (1.10.5)
       mini_portile2 (~> 2.4.0)
     nokogumbo (2.0.1)
@@ -268,7 +269,8 @@ GEM
     pry-rails (0.3.9)
       pry (>= 0.10.4)
     public_suffix (3.0.3)
-    puma (3.12.2)
+    puma (4.3.1)
+      nio4r (~> 2.0)
     r2 (0.2.7)
     rack (2.0.7)
     rack-mini-profiler (1.1.3)