Change API Documenter (BETA) annotation to (Public Preview)

[dlarocque]

The API documenter annotates @beta APIs with (BETA) in the docs. We use @beta to show that an API is in public preview, so we should update the annotation to be (Public Preview).

This affects two Firestore features; MultiDB (@tom-andersen #7356) and Indexes (@wu-hui #6510) - are these two APIs meant for public preview?

Example of (BETA) annotation in the docs:

[changeset-bot]

⚠️ No Changeset found

Latest commit: 4b29dd2

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[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (554c7bd) and merge commit (265ace0).

Test Logs

• Base (554c7bd): https://github.com/firebase/firebase-js-sdk/actions/runs/13291543556
• Merge (265ace0): https://github.com/firebase/firebase-js-sdk/actions/runs/13313365832

1. https://storage.googleapis.com/firebase-sdk-metric-reports/AMGU2SIuYA.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (554c7bd) and merge commit (265ace0).

Test Logs

• Base (554c7bd): https://github.com/firebase/firebase-js-sdk/actions/runs/13291543556
• Merge (265ace0): https://github.com/firebase/firebase-js-sdk/actions/runs/13313365832

1. https://storage.googleapis.com/firebase-sdk-metric-reports/ldZFrrrGHl.html