Fix Thumbnail Generation (cyberbotics#6598)

* Specify encoding for opening files.
Required to support emojis

* Updated changelog

* Revert "Updated changelog"

This reverts commit 8ef5539.

docs/generate_thumbnails.py

@@ -23,10 +23,10 @@
 
 def search_and_replace(filename, fromString, toString):
     """Search and replace string in a file."""
-    with open(filename, 'r') as file:
+    with open(filename, 'r', encoding="utf-8") as file:
         data = file.read()
     data = data.replace(fromString, toString)
-    with open(filename, 'w') as file:
+    with open(filename, 'w', encoding="utf-8") as file:
         file.write(data)