Rename action `functionRef` property

[cdavernas]

What would you like to be added:

Now that we split eventRef into two new subscribe and publish properties, we should rename the functionRef into something like call, so that we aim at consistently using imperative tense. We should also rename the functionRef's refName property into function.

actions:
- name: my-action
  call:
    function: greet
    arguments:
      greetings: Hello, World!

Why is this needed:

• Use cleaner, simpler terminology accross the spec, and avoid at all cost using abbreviations.
• Align with https://github.com/serverlessworkflow/specification/pull/826.

[github-actions[bot]]

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