templates,docs: leave templates in v0.2 (#27952)

all template installs will now have to declare `--branch v0.2` to make
clear they aren't compatible with langchain 0.3 (most have a pydantic v1
setup). e.g.

```
langchain-cli app add pirate-speak --branch v0.2
```

.github/scripts/check_diff.py

@@ -307,7 +307,7 @@ if __name__ == "__main__":
                 f"Unknown lib: {file}. check_diff.py likely needs "
                 "an update for this new library!"
             )
-        elif any(file.startswith(p) for p in ["docs/", "templates/", "cookbook/"]):
+        elif any(file.startswith(p) for p in ["docs/", "cookbook/"]):
             if file.startswith("docs/"):
                 docs_edited = True
             dirs_to_run["lint"].add(".")