[DOCS-4022] docs: Update README for Event Feeds GA

[jrodewig]

Description

Updates the README to remove references to Event Feeds being in beta.

Motivation and context

Prepare the README for GA release.

See DOCS-4022.

How was the change tested?

Screenshots (if appropriate):

Change types

• • Bug fix (non-breaking change that fixes an issue)
• • New feature (non-breaking change that adds functionality)
• • Breaking change (backwards-incompatible fix or feature)

Checklist:

• • My code follows the code style of this project.
• • My change requires a change to Fauna documentation.
• • My change requires a change to the README, and I have updated it accordingly.

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