Roll tensorflow python package back to 2.18

Tensorflow 2.19 is introducing breaking changes with respect to IREE.
See https://github.com/iree-org/iree/issues/19777 for more info.

Tested:
https://louhi.dev/5958780716318720/execution-detail/6644989968121856
https://louhi.dev/5958780716318720/execution-detail/6733312850001920

Bypass-Presubmit-Reason: no presubmit, tested manually
BUG: 395688332
Change-Id: I0d605b7b9cef8d5882e33381ae2e38d5f0b625c8
diff --git a/python-requirements.txt b/python-requirements.txt
index eec4f73..162a3c1 100644
--- a/python-requirements.txt
+++ b/python-requirements.txt
@@ -44,6 +44,6 @@
 scipy
 sel4-deps
 setuptools
-tf-nightly==2.19.0.dev20250209
+tensorflow==2.18
 wget
 yapf