build(deps): update boring requirement from 4.15.7 to 4.15.8 (#468)

See sfackler/rust-openssl#2360 and https://nvd.nist.gov/vuln/detail/CVE-2025-24898. From the rust-openssl PR:
> `SSL_select_next_proto` can return a pointer into either the client or server buffers, but the type signature of the function previously only bound the output buffer to the client buffer. This can result in a UAF in situations where the server slice does not point to a long-lived allocation.

Thanks to Matt Mastracci for reporting this issue.

Author: 0x676e67

Cargo.toml

@@ -114,8 +114,8 @@ ipnet = "2.11.0"
 arc-swap = "1.7.0"
 
 ## boring-tls
-boring2 = { version = "4.15.7", features = ["pq-experimental", "cert-compression"] }
-tokio-boring2 = { version = "4.15.7", features = ["pq-experimental"] }
+boring2 = { version = "4.15.8", features = ["pq-experimental", "cert-compression"] }
+tokio-boring2 = { version = "4.15.8", features = ["pq-experimental"] }
 linked_hash_set = "0.1"
 
 # Optional deps...