ci: Use beta-plugin-slug in autorelease action

[anomiex]

Proposed changes:

If no .extra.autorelease.slug and no .extra.wp-plugin-slug exist, but .extra.beta-plugin-slug does, use that.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

p1718306055507569/1718304573.075299-slack-C05Q5HSS013

Does this pull request change what data or activity we track or use?

no

Testing instructions:

• 🤷

[github-actions]

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[tbradsha]

Looks good to me.