Backend Tensorflow 1.x: replace tf.layers.dense to tf.keras.layers.Dense

[vl-dud]

Switch to the modern TF style, which avoids such warnings:

UserWarning: `tf.layers.dense` is deprecated and will be removed in a future version. Please use `tf.keras.layers.Dense` instead.

[lululxvi]

Please see below for the regularizaion:

deepxde/deepxde/nn/tensorflow_compat_v1/nn.py

Line 14 in ba8e824

# tf.layers.dense() is not available for TensorFlow 2.16+ with Keras 3. The

deepxde/deepxde/nn/tensorflow_compat_v1/fnn.py

Line 118 in ba8e824

self.regularization_loss += tf.math.add_n(dense.losses)

[vl-dud]

You are right, the loss of regularization was ignored. I have corrected it.