Auto merge of #3364 - JohnTitor:oauth2-4.0.0-alpha.6, r=Turbo87

Upgrade `oauth2` to 4.0.0-alpha.6

The changes between alpha.3 and alpha.6: ramosbugs/oauth2-rs@4.0.0-alpha.3...4.0.0-alpha.6

All the breaking changes mentioned on https://github.com/ramosbugs/oauth2-rs/releases and the changes themselves don't seem to affect our codebase.
Also pin its version to prevent updating by accident when running `cargo update`.

r? `@jtgeibel`

Author: bors

Cargo.lock

@@ -67,7 +67,7 @@ indexmap = "1.0.2"
 jemallocator = { version = "0.3", features = ['unprefixed_malloc_on_supported_platforms', 'profiling'] }
 lettre = { version = "0.10.0-alpha.1", default-features = false, features = ["file-transport", "smtp-transport", "native-tls", "hostname", "builder"] }
 license-exprs = "1.6"
-oauth2 = { version = "4.0.0-alpha.3", default-features = false, features = ["reqwest"] }
+oauth2 = { version = "=4.0.0-alpha.6", default-features = false, features = ["reqwest"] }
 parking_lot = "0.11"
 rand = "0.8"
 reqwest = { version = "0.11", features = ["blocking", "gzip", "json"] }