chore: Fix docs on SslRef::replace_ex_data (#40)

* RTG-3333 Support X25519MLKEM768 by default, but don't sent it as client

X25519MLKEM768 is the standardised successor of the preliminary
X25519Kyber768Draft00. Latest browsers have switched to X25519MLKEM768.
Cloudflare supports both on the edge.

We've had support for X25519MLKEM768 in this crate for a while, but
didn't enable by default. We're now enabling serverside support by
default. We also let clients advertise support when set
to kx-client-pq-supported.

We don't enable support by default yet for clients set to
kx-client-pq-preferred, as that would cause an extra round-trip due to
HelloRetryRequest if the server doesn't support X25519MLKEM768 yet.

BoringSSL against which we build must support X25519MLKEM768, otherwise
this will fail.

* replace once_cell with LazyLock

We can drop the once_cell dependency since the same functionality is
implemented in std now.

Requires bumping MSRV to 1.80.

* fix manual_c_str_literals clippy warning

* chore: Fix docs on SslRef::replace_ex_data

---------

Co-authored-by: Bas Westerbaan <bas@cloudflare.com>
Co-authored-by: Alessandro Ghedini <alessandro@cloudflare.com>
Co-authored-by: Evan Rittenhouse <erittenhouse@cloudflare.com>

boring/src/ssl/mod.rs

@@ -3649,7 +3649,7 @@ impl SslRef {
     /// This can be used to provide data to callbacks registered with the context. Use the
     /// `Ssl::new_ex_index` method to create an `Index`.
     ///
-    /// Any previous value will be dropped and replaced by the new one.
+    /// The previous value, if any, will be returned.
     #[corresponds(SSL_set_ex_data)]
     pub fn replace_ex_data<T>(&mut self, index: Index<Ssl, T>, data: T) -> Option<T> {
         if let Some(old) = self.ex_data_mut(index) {