TF: Add sigmoid activation function
# What does this PR do?

Fixes #16810 -- adds the sigmoid activation function to the TF activation functions. 

Also sorts the activation functions in the enum-like dict and in the tests, to quickly identify missing functions.
_The documentation is not available anymore as the PR was closed or merged._<|||||>As an aside, GELU is a core Keras activation now, although we might have to wait until we can move our minimum version before we can switch to using it instead of our own implementations. Other than that, this looks great!