fix: replace UI.warning with UI.important in changelog_helper

UI.warning was removed in Fastlane 2.228.0. Replace with UI.important
which is the recommended alternative for non-critical warnings.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: graycreate

fastlane/changelog_helper.rb

@@ -36,7 +36,7 @@ def self.extract_changelog(version)
     end
 
     if start_index.nil?
-      UI.warning("Version #{version} not found in CHANGELOG.md")
+      UI.important("Version #{version} not found in CHANGELOG.md")
       UI.message("Available versions:")
       lines.each do |line|
         if line.match?(/^##\s+v?\d+\.\d+\.\d+/)
@@ -66,7 +66,7 @@ def self.extract_changelog(version)
       .strip
 
     if changelog.empty?
-      UI.warning("No changelog content found for version #{version}")
+      UI.important("No changelog content found for version #{version}")
       return "Bug fixes and improvements"
     end