ttrpc-rust is using a couple of outdated crates as dependencies

[egernst]

Utilizing cargo-audit, we can see that Prost needs updating. Let's update to >= 0.8, and then rev this crate version.

Audit output:

$ cargo-audit audit
    Fetching advisory database from `https://github.com/RustSec/advisory-db.git`
      Loaded 317 security advisories (from /home/eernst/.cargo/advisory-db)
    Updating crates.io index
    Scanning Cargo.lock for vulnerabilities (55 crate dependencies)
Crate:         prost
Version:       0.5.0
Title:         Parsing a specially crafted message can result in a stack overflow
Date:          2020-01-16
ID:            RUSTSEC-2020-0002
URL:           https://rustsec.org/advisories/RUSTSEC-2020-0002
Solution:      Upgrade to >=0.6.1
Dependency tree: 
prost 0.5.0
├── ttrpc-compiler 0.4.0
├── prost-types 0.5.0
│   ├── ttrpc-compiler 0.4.0
│   └── prost-build 0.5.0
│       └── ttrpc-compiler 0.4.0
└── prost-build 0.5.0

Crate:         prost-types
Version:       0.5.0
Title:         Conversion from `prost_types::Timestamp` to `SystemTime` can cause an overflow and panic
Date:          2021-07-08
ID:            RUSTSEC-2021-0073
URL:           https://rustsec.org/advisories/RUSTSEC-2021-0073
Solution:      Upgrade to >=0.8.0
Dependency tree: 
prost-types 0.5.0
├── ttrpc-compiler 0.4.0
└── prost-build 0.5.0
    └── ttrpc-compiler 0.4.0

Crate:         failure
Version:       0.1.8
Warning:       unmaintained
Title:         failure is officially deprecated/unmaintained
Date:          2020-05-02
ID:            RUSTSEC-2020-0036
URL:           https://rustsec.org/advisories/RUSTSEC-2020-0036
Dependency tree: 
failure 0.1.8
├── which 2.0.1
│   └── prost-build 0.5.0
│       └── ttrpc-compiler 0.4.0
└── prost-derive 0.5.0
    └── prost 0.5.0
        ├── ttrpc-compiler 0.4.0
        ├── prost-types 0.5.0
        │   ├── ttrpc-compiler 0.4.0
        │   └── prost-build 0.5.0
        └── prost-build 0.5.0

error: 2 vulnerabilities found!
warning: 1 allowed warning found