CI: use arrays for Puma versions

The [String, Float, Float] array syntax of
min-library-version-as-string, min-ruby-version-as-float, and (optional)
max-ruby-version-as-float can be used to express version constraints

test/multiverse/suites/rack/Envfile

@@ -6,19 +6,14 @@ instrumentation_methods :chain, :prepend
 
 # The Rack suite also tests Puma::Rack::Builder
 # Which is why we also control Puma tested versions here
-PUMA_VERSIONS = [
-  'nil',
-  '5.6.4',
-  '4.3.12',
-  '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
+PUMA_VERSIONS = [
+  [nil, 2.4],
+  ['5.6.4', 2.4, 3.2],
+  ['4.3.12', 2.4, 3.2],
+  ['3.12.6', 2.4, 3.2]
+]
 
 def gem_list(puma_version = nil)
   <<~RB