releng: use pyproject.toml not towncrier.toml

According to the towncrier documentation
https://towncrier.readthedocs.io/en/23.11.0/tutorial.html

> towncrier keeps its config in the PEP-518 pyproject.toml or a
> towncrier.toml file. If the latter exists, it takes precedence.

In our case, we don't have towncrier.toml, only pyproject.toml

releng/generate-release-notes

@@ -29,7 +29,7 @@ def _limited_config(opts):
         return []
     with tempfile.NamedTemporaryFile(delete=False, dir=".") as config_fd:
         opts.temp_config = config_fd.name
-        with open("towncrier.toml", "rb") as fd:
+        with open("pyproject.toml", "rb") as fd:
             for line in fd.readlines():
                 if b'{ name' not in line:
                     config_fd.write(line)