Bump protobuf

[jamesreprise]

What is the goal of this PR?

We've bumped our protobuf dependency in response to the following issue:

• GCP and protobuf requirement updates #270

What are the changes implemented in this PR?

We've changed our pinned protobuf version from 3.15.5 to 3.20.1.

[typedb-bot]

PR Review Checklist

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Trivial Change

• This change is trivial and does not require a code or architecture review.

Code

• Packages, classes, and methods have a single domain of responsibility.
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