Rename usages of "master" branch to "main"

[hsubox76]

This is to match the coordinated effort across all Firebase platform SDKs. The change has already been made in the Apple and Android repos.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 303efa2

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (a9f8440) and merge commit (7c0bf1c).

Test Logs

• Base (a9f8440): https://github.com/firebase/firebase-js-sdk/actions/runs/10116063249
• Merge (7c0bf1c): https://github.com/firebase/firebase-js-sdk/actions/runs/10116898250

1. https://storage.googleapis.com/firebase-sdk-metric-reports/AbXNWbgXl0.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (a9f8440) and merge commit (7c0bf1c).

Test Logs

• Base (a9f8440): https://github.com/firebase/firebase-js-sdk/actions/runs/10116063249
• Merge (7c0bf1c): https://github.com/firebase/firebase-js-sdk/actions/runs/10116898250

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