CI: Ruby v3.3.0+ requires Puma v6.4.0+

Now that Puma v6.4.0 is out, we can resume testing with "nil" (latest)
on Ruby v3.3.0, but we need to prevent Ruby v3.3.0 and future rubies
from ever testing Pumas older than v6.4.0.

test/multiverse/suites/rack/Envfile

@@ -13,6 +13,13 @@ PUMA_VERSIONS = [
   '3.12.6'
 ]
 
+# Ruby v3.3.0+ requires Puma v6.4.0+
+# https://github.com/puma/puma/commit/188f5da1920ff99a8689b3e9b46f2f26b7c62d66
+if RUBY_VERSION >= Gem::Version.new('3.3.0')
+  puma_min = Gem::Version.new('6.4.0')
+  PUMA_VERSIONS.reject! { |v| !v.eql?('nil') && Gem::Version.new(v) < puma_min }
+end
+
 def gem_list(puma_version = nil)
   <<~RB
     gem 'puma'#{puma_version}