Pin TF for 2.21 branch (#7128)

This is the 1/3 changes for release of 2.21.0

Author: psamanoelton

.github/workflows/ci.yml

@@ -30,7 +30,7 @@ env:
   BUILDTOOLS_VERSION: '3.0.0'
   BUILDIFIER_SHA256SUM: 'e92a6793c7134c5431c58fbc34700664f101e5c9b1c1fcd93b97978e8b7f88db'
   BUILDOZER_SHA256SUM: '3d58a0b6972e4535718cdd6c12778170ea7382de7c75bc3728f5719437ffb84d'
-  TENSORFLOW_VERSION: 'tf-nightly'
+  TENSORFLOW_VERSION: 'tf-nightly==2.21.0.dev20260203'
 
 defaults:
   run: