Upgrade puma in the initial-placeholder app, to satisfy Dependabot

So, Dependabot correctly reported that this version of puma is
vulernable, which I fixed in the main app already—but I didn't notice we
also use that version in this cute tiny placeholder app we use early in
the deployment process.

There's not a real security need to upgrade this, as this placeholder
app has no access to useful data when it is run, but I think it's better
to resolve this by fixing it than by silencing Dependabot! May as well!

deploy/files/initial-placeholder/Gemfile

@@ -1,2 +1,2 @@
 source 'https://rubygems.org'
-gem 'puma', '~> 6.3'
+gem 'puma', '~> 6.3', '>= 6.3.1'

deploy/files/initial-placeholder/Gemfile.lock

@@ -2,7 +2,7 @@ GEM
   remote: https://rubygems.org/
   specs:
     nio4r (2.5.9)
-    puma (6.3.0)
+    puma (6.4.0)
       nio4r (~> 2.0)
 
 PLATFORMS
@@ -10,7 +10,7 @@ PLATFORMS
   x86_64-linux
 
 DEPENDENCIES
-  puma (~> 6.3)
+  puma (~> 6.3, >= 6.3.1)
 
 BUNDLED WITH
    2.4.18