Call ERB.new correctly depending on the RUBY_VERSION

* On Ruby 3.0.x and older, call ERB.new(template_code, nil, "-")
* On Ruby 3.1.0 and newer, call ERB.new(template_code, trim_mode: "-")

Fixes #1894

lib/fpm/package.rb

@@ -331,11 +331,12 @@ def template(path)
     template_path = File.join(template_dir, path)
     template_code = File.read(template_path)
     logger.info("Reading template", :path => template_path)
-    erb = ERB.new(template_code, nil, "-")
+    erb = erbnew(template_code)
     erb.filename = template_path
     return erb
   end # def template
 
+
   #######################################
   # The following methods are provided to
   # easily override particular substitut-
@@ -518,7 +519,7 @@ def write_scripts
   # flag), then apply it as an ERB template.
   def script(script_name)
     if attributes[:template_scripts?]
-      erb = ERB.new(scripts[script_name], nil, "-")
+      erb = erbnew(scripts[script_name])
       # TODO(sissel): find the original file name for the file.
       erb.filename = "script(#{script_name})"
       return erb.result(binding)

lib/fpm/util.rb

@@ -406,6 +406,29 @@ def expand_pessimistic_constraints(constraint)
   def logger
     @logger ||= Cabin::Channel.get
   end # def logger
+
+  def erbnew(template_code)
+    # In Ruby 2.6(?), Ruby changed how ERB::new is invoked.
+    # First, it added keyword args like `ERB.new(..., trim_mode: "-")`
+    # Later, it deprecated then removed the safe_level feature.
+    # As of Ruby 3.1, warnings are printed at runtime when ERB.new is called with the old syntax.
+    # Ruby 2.5 and older does not support the ERB.new keyword args.
+    #
+    # My tests showed:
+    # * Ruby 2.3.0 through 3.0 work correctly with the old syntax.
+    # * Ruby 3.1.0 and newer (at time of writing, Ruby 3.2) require the new syntax
+    # Therefore, in order to support the most versions of ruby, we need to do a version check
+    # to invoke ERB.new correctly and without printed warnings.
+    # References: https://github.com/jordansissel/fpm/issues/1894
+    # Honestly, I'm not sure if Gem::Version is correct to use in this situation, but it works.
+    if Gem::Version.new(RUBY_VERSION) < Gem::Version.new("3.1.0")
+      # Ruby 3.0.x and older
+      return ERB.new(template_code, nil, "-")
+    else
+      # Ruby 3.1.0 and newer
+      return ERB.new(template_code, trim_mode: "-")
+    end
+  end
 end # module FPM::Util
 
 require 'fpm/util/tar_writer'