Update migrate/tensorboard.ipynb with Time Series as default dashboard.

With the TensorBoard 2.10 release the "Time Series" dashboard will be the default dashboard for logs with scalars, histograms, or images events. We are updating external-facing documentation to reflect this change.

Mirroring work in the TensorBoard repository (tensorflow/tensorboard#5861) this change updates notebook documentation to use the "Time Series" dashboard where previously it used the "Scalars" dashboard.

Notably:
* Updated images to use "Time Series" and set them to 1600x800 pixels to be consistent with other changes.
* Some of the models being created now give explicit names to the layers. This is so that the tags displayed in the Time Series dashboard maintain a logical ordering of scalars ahead of histograms and images.
PiperOrigin-RevId: 508444818

Author: tensorflower-gardener

site/en/guide/migrate/images/tensorboard_TF1.png

@@ -218,10 +218,10 @@
         "\n",
         "def create_model():\n",
         "  return tf.keras.models.Sequential([\n",
-        "    tf.keras.layers.Flatten(input_shape=(28, 28)),\n",
-        "    tf.keras.layers.Dense(512, activation='relu'),\n",
-        "    tf.keras.layers.Dropout(0.2),\n",
-        "    tf.keras.layers.Dense(10, activation='softmax')\n",
+        "    tf.keras.layers.Flatten(input_shape=(28, 28), name='layers_flatten'),\n",
+        "    tf.keras.layers.Dense(512, activation='relu', name='layers_dense'),\n",
+        "    tf.keras.layers.Dropout(0.2, name='layers_dropout'),\n",
+        "    tf.keras.layers.Dense(10, activation='softmax', name='layers_dense_2')\n",
         "  ])\n",
         "\n",
         "model = create_model()\n",