Fix docs/rtdb/objects.md to reflect 5.0.0 changes and deprecations

[c-goosen]

Checklist

• Issue number for this PR: #nnn (required)
• Docs included?: (yes/no; required for all API/functional changes)
• Test units included?: (yes/no; required)
• In a clean directory, yarn install, yarn test run successfully? (yes/no; required)

Description

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.
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[c-goosen]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[jamesdaniels]

Thanks!

[c-goosen]

No problem. Will fix as I work through the project.