Update get_available_activation_fns to match get_activation_fn

[DavidMChan]

Before submitting

• [x] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
• [x] Did you read the contributor guideline?
• [x] Did you make sure to update the docs?
• [x] Did you write any new necessary tests?

What does this PR do?

Fixes a discrepancy between the get_activation_fn() method and the get_available_activation_fns() method (enabling "swish" and "relu_squared" as available).

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