Revert "Fix Sphinx `add_lexer()` deprecation in the GDScript extension (#4594)"

This reverts commit 7c71a2898b90202b545aad14cbae6512414bb451.

It broke the build.

_extensions/gdscript.py

@@ -339,7 +339,7 @@ class GDScriptLexer(RegexLexer):
 
 
 def setup(sphinx):
-    sphinx.add_lexer("gdscript", GDScriptLexer)
+    sphinx.add_lexer("gdscript", GDScriptLexer())
 
     return {
         "parallel_read_safe": True,