chore: use gapic-generator-python 0.63.4

chore: fix snippet region tag format
chore: fix docstring code block formatting
PiperOrigin-RevId: 430730865

WORKSPACE

@@ -225,7 +225,7 @@ load("@rules_python//python:pip.bzl", "pip_repositories")
 
 pip_repositories()
 
-_gapic_generator_python_version = "0.63.2"
+_gapic_generator_python_version = "0.63.4"
 
 http_archive(
     name = "gapic_generator_python",