rubocop: Drop "fn" from Naming/MethodParameterName allowlist

- I originally thought this was short for "function", but upon closer
  inspection all its usages are to do with filenames. So, use "filename",
  it's clearer.

Library/.rubocop.yml

@@ -198,7 +198,6 @@ Naming/MethodParameterName:
       "e",
       "f",
       "ff",
-      "fn",
       "o",
       "p",
       "pr",

Library/Homebrew/cask/download.rb

@@ -57,7 +57,7 @@ module Cask
       downloader.basename
     end
 
-    def verify_download_integrity(fn)
+    def verify_download_integrity(filename)
       if @cask.sha256 == :no_check
         opoo "No checksum defined for cask '#{@cask}', skipping verification."
         return
@@ -65,13 +65,13 @@ module Cask
 
       begin
         ohai "Verifying checksum for cask '#{@cask}'" if verbose?
-        fn.verify_checksum(@cask.sha256)
+        filename.verify_checksum(@cask.sha256)
       rescue ChecksumMissingError
         opoo <<~EOS
-          Cannot verify integrity of '#{fn.basename}'.
+          Cannot verify integrity of '#{filename.basename}'.
           No checksum was provided for this cask.
           For your reference, the checksum is:
-            sha256 "#{fn.sha256}"
+            sha256 "#{filename.sha256}"
         EOS
       end
     end

Library/Homebrew/formula.rb

@@ -2281,8 +2281,8 @@ class Formula
   end
 
   # @private
-  def verify_download_integrity(fn)
-    active_spec.verify_download_integrity(fn)
+  def verify_download_integrity(filename)
+    active_spec.verify_download_integrity(filename)
   end
 
   # @private

Library/Homebrew/readall.rb

@@ -118,9 +118,9 @@ module Readall
 
     private
 
-    def syntax_errors_or_warnings?(rb)
+    def syntax_errors_or_warnings?(filename)
       # Retrieve messages about syntax errors/warnings printed to `$stderr`.
-      _, err, status = system_command(RUBY_PATH, args: ["-c", "-w", rb], print_stderr: false)
+      _, err, status = system_command(RUBY_PATH, args: ["-c", "-w", filename], print_stderr: false)
 
       # Ignore unnecessary warning about named capture conflicts.
       # See https://bugs.ruby-lang.org/issues/12359.

Library/Homebrew/resource.rb

@@ -187,17 +187,17 @@ class Resource
     download
   end
 
-  def verify_download_integrity(fn)
-    if fn.file?
-      ohai "Verifying checksum for '#{fn.basename}'" if verbose?
-      fn.verify_checksum(checksum)
+  def verify_download_integrity(filename)
+    if filename.file?
+      ohai "Verifying checksum for '#{filename.basename}'" if verbose?
+      filename.verify_checksum(checksum)
     end
   rescue ChecksumMissingError
     opoo <<~EOS
-      Cannot verify integrity of '#{fn.basename}'.
+      Cannot verify integrity of '#{filename.basename}'.
       No checksum was provided for this resource.
       For your reference, the checksum is:
-        sha256 "#{fn.sha256}"
+        sha256 "#{filename.sha256}"
     EOS
   end
 

Library/Homebrew/software_spec.rb

@@ -286,7 +286,7 @@ class HeadSoftwareSpec < SoftwareSpec
     @resource.version = Version.create("HEAD")
   end
 
-  def verify_download_integrity(_fn)
+  def verify_download_integrity(_filename)
     # no-op
   end
 end