tensorflow: flip 2.21 configs to production release (#6375)

Promotes TF 2.21 SageMaker configs from gamma to production after
successful gamma validation. Autorelease will push to the prod ECR
(061416347176) and sync to public.ecr.aws/deep-learning-containers/tensorflow-training.

Changes per config (both cpu + cuda):
- public_registry: false → true (publish to public gallery)
- environment: "gamma" → "production"

private_registry stays true (still publish to private prod ECR).
enable_soci stays true (already on from gamma retest cycle).

Co-authored-by: Bhanu Teja Goshikonda <bhanugk@amazon.com>

Author: bhanutejagk

.github/config/image/tensorflow/2.21-sagemaker-cpu.yml

@@ -23,7 +23,7 @@ build:
 release:
   release: true
   force_release: false
-  public_registry: false
+  public_registry: true
   private_registry: true
   enable_soci: true
-  environment: "gamma"
+  environment: "production"

.github/config/image/tensorflow/2.21-sagemaker-cuda.yml

@@ -25,7 +25,7 @@ build:
 release:
   release: true
   force_release: false
-  public_registry: false
+  public_registry: true
   private_registry: true
   enable_soci: true
-  environment: "gamma"
+  environment: "production"