Update protobuf requirement from 2.0 to 3.1

[dependabot[bot]]

Updates the requirements on protobuf to permit the latest version.

Changelog

Sourced from protobuf's changelog.

[3.1.0] - 2022-06-22

• Reflection API to clear fields

[3.0.3] - 2022-05-31

• optional fields in proto3 are now handled correctly in protoc-gen-rust plugin for protoc

[3.0.2] - 2022-05-06

• Sort modules in generated mod.rs

[3.0.1] - 2022-05-01

• More up to date readme.

[3.0.0] - 2022-05-01

• New stable version released.

Before version 3

Changelog before version 3

Commits

• c65c7d5 Bump version to 3.1.0
• 70cd66e Move clear_field reflection test
• 7f70813 Reflection clear_field tests
• d4aa230 Allow clearing field via reflection
• a9ca7ed Bump version to 3.1.0-pre
• 09c9c62 Bump version to 3.0.3
• 276a3cf set supported features (#626)
• 3585c2c Bump version to 3.0.2
• 6ff997e Fix generated mod.rs to pass cargo fmt -- --check
• 2bf4397 Erase changelog
• Additional commits viewable in compare view

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[Xuanwo]

@dependabot rebase

[Xuanwo]

@YangKeao Are there other blocks from this PR that get merged? I'm willing to fix the API breaks.

[YangKeao]

@YangKeao Are there other blocks from this PR that get merged? I'm willing to fix the API breaks.

@Xuanwo The 3.x version rust-protobuf seems not to be used by many users. I'm afraid it will become a huge break for existing user.

See https://crates.io/crates/protobuf "Downloads over the last 90 days"

[Xuanwo]

Let's close this PR until we think protobuf v3 is mature enough.

[dependabot[bot]]

OK, I won't notify you again about this release, but will get in touch when a new version is available. If you'd rather skip all updates until the next major or minor version, let me know by commenting @dependabot ignore this major version or @dependabot ignore this minor version. You can also ignore all major, minor, or patch releases for a dependency by adding an ignore condition with the desired update_types to your config file.

If you change your mind, just re-open this PR and I'll resolve any conflicts on it.