protobuf-python has a potential Denial of Service issue

[benaslater]

Describe the bug
Hello! We build a project (animal-ai) using ml-agents, and have recently had a security warning about versions of protobuf below 4.25.8 (See this, or this).

I guess it's not a big issue as ml-agents protobuf usage doesn't deal with untrusted inputs, but it would be good tidy this up - would it be possible to bump the protobuf version used in ml-agents?

To Reproduce
We pin the protobuf version explicitly (ref), which is why we get the alert. (We pinned before my time, I'm not sure exactly why we need to pin instead of inheriting the version from you).

I'm not sure why we get it and you don't, as your specified protobuf python versions appear to be within the affected range (ref).

Console logs / stack traces
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Screenshots
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Environment (please complete the following information):
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