Reverts SE and experimental TF tests to use Keras 2 instead of 3

[EricLeFort]

Description

Reverts to Keras 2 from Keras 3 for the tf.se and tf.exp tests.

Tests

Please describe the tests that you ran on TPUs to verify changes.

Instruction and/or command lines to reproduce your tests:
./scripts/run-oneshot.sh -t tf.se.nightly-keras-api-connection-v2-8-1vm

List links for your tests (use go/shortn-gen for any internal link):
Succeeding run

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• [ X ] I have performed a self-review of my code.
• [ X ] I have necessary comments in my code, particularly in hard-to-understand areas.
• [ X ] I have run one-shot tests and provided workload links above if applicable.
• [ X ] I have made or will make corresponding changes to the doc if needed.

[allenwang28]

thanks!