dependencies: get rid of custom fork of openidconnect-rs

There's a published 4.0.0-alpha.2 version that compiles and doesn't
require the custom changes we needed.

Part of #3742.

Author: bnjbvr

Cargo.lock

@@ -127,7 +127,7 @@ tokio = { workspace = true, features = ["macros"] }
 
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 backoff = { version = "0.4.0", features = ["tokio"] }
-openidconnect = { git = "https://github.com/poljar/openidconnect-rs", rev = "c7e1dc31b83dd7559125984bfd36b9c0f191585e", optional = true }
+openidconnect = { version = "4.0.0-alpha.2", optional = true }
 # only activate reqwest's stream feature on non-wasm, the wasm part seems to not
 # support *sending* streams, which makes it useless for us.
 reqwest = { workspace = true, features = ["stream"] }