Cannot regenerate protobuf source code using instructions in the documentation #8602

Describe the bug

I'm trying to regenerate protobuf source code by running regen.sh but it fails with an error:

$ ./regen.sh 
    Finished dev [unoptimized + debuginfo] target(s) in 0.99s
     Running `/home/andy/git/apache/arrow-datafusion/target/debug/gen`
Error: "protobuf compilation failed: protoc failed: datafusion.proto: This file contains proto3 optional fields, but --experimental_allow_proto3_optional was not set.\n"

I am using libprotoc 3.12.4 as suggested by the docs.

I see a comment in datafusion.proto ... "optional" keyword is stable in protoc 3.15, so perhaps our docs are out of date and we now require 3.15? If so, we should update the docs.

To Reproduce

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Expected behavior

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Additional context

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