Revert webpki version bump to keep compatability with tokio-rustls

[ETCaton]

The latest webpki version bump results in some compilation failures when compiling sqlx-core when compiling with tokio-rustls, which depends on 0.21.4. For example:

error[E0308]: mismatched types
  --> /home/etcaton/.cargo/registry/src/github.com-1ecc6299db9ec823/sqlx-core-0.5.3/src/net/tls/rustls.rs:73:39
   |
72 |         match verifier.verify_server_cert(roots, presented_certs, dns_name, ocsp_response) {
   |               ---------------------------------------------------------------------------- this expression has type `std::result::Result<ServerCertVerified,
TLSError>`
73 |             Err(TLSError::WebPKIError(webpki::Error::CertNotValidForName)) => {
   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected enum `webpki::error::Error`, found enum `webpki::Error`
   |
   = note: perhaps two different versions of crate `webpki` are being used?

https://github.com/tokio-rs/tls/blob/794659740dcc399f79058c4eba325ffd97474c7b/tokio-rustls/Cargo.toml#L17

webpki v0.21.4
├── rustls v0.19.1
│   ├── sqlx-core v0.5.3 (https://github.com/ETCaton/sqlx#15c15019)
│   │   ├── sqlx v0.5.3 (https://github.com/ETCaton/sqlx#15c15019)
│   │   │   └── <my crate>
│   │   └── sqlx-macros v0.5.3 (proc-macro) (https://github.com/ETCaton/sqlx#15c15019)
│   │       └── sqlx v0.5.3 (https://github.com/ETCaton/sqlx#15c15019) (*)
│   └── tokio-rustls v0.22.0
│       └── sqlx-rt v0.5.3 (https://github.com/ETCaton/sqlx#15c15019)
│           ├── sqlx-core v0.5.3 (https://github.com/ETCaton/sqlx#15c15019) (*)
│           └── sqlx-macros v0.5.3 (proc-macro) (https://github.com/ETCaton/sqlx#15c15019) (*)
├── sqlx-core v0.5.3 (https://github.com/ETCaton/sqlx#15c15019) (*)
├── tokio-rustls v0.22.0 (*)
└── webpki-roots v0.21.1
    └── sqlx-core v0.5.3 (https://github.com/ETCaton/sqlx#15c15019) (*)

[ETCaton]

Perhaps #960 would alleviate this, but I didn't see an existing issue or PR for this

[mehcode]

Thanks. Our CI is a bit futzy and I had forgotten to check rustls compilation before the release. Will push a .4 out soon.