Add --upgrade to keras install

[sampathweb]

If the user's env already has keras via tensorflow, pip install keras doesn't do anything since its already installed. So adding --upgrade.

Also, !pip install --upgrade keras>=3 fails in Colab since pip checks for dependencies and shows error -

ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
tensorflow 2.14.0 requires keras<2.15,>=2.14.0, but you have keras 3.0.1 which is incompatible.

So, we will have to use pip install --upgrade keras if we want to upgrade to version 3.