Trim trailing whitespace from *.md files

Fixes the error:

    Run bin/check-changelogs ./rails
    ............F

    Offenses:

    ./rails/railties/CHANGELOG.md:4 Trailing whitespace detected.

CODE_OF_CONDUCT.md

@@ -9,4 +9,3 @@ https://rubyonrails.org/conduct
 For a history of updates, see the page history here:
 
 https://github.com/rails/website/commits/main/_pages/conduct.html
-

guides/CHANGELOG.md

@@ -1,3 +1 @@
-
-
 Please check [7-0-stable](https://github.com/rails/rails/blob/7-0-stable/guides/CHANGELOG.md) for previous changes.

guides/source/7_1_release_notes.md

@@ -1,4 +1,3 @@
-
 **DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON https://guides.rubyonrails.org.**
 
 Ruby on Rails 7.1 Release Notes