Use openssl@3 when building on macOS

There is an issue building a project depending on `swift-kafka-client`
on macOS, with `libressl` installed. Explicitly switching to `openssl`
on macOS remediates the issue with all tried combinations.

Resolves #191

Author: mimischi

Package.swift

@@ -102,7 +102,7 @@ let package = Package(
             name: "COpenSSL",
             pkgConfig: "openssl",
             providers: [
-                .brew(["libressl"]),
+                .brew(["openssl"]),
                 .apt(["libssl-dev"]),
             ]
         ),

README.md

@@ -243,6 +243,7 @@ It has source files that are excluded in `Package.swift`.
 ### Dependencies
 
 `librdkafka` depends on `openssl`, meaning that `libssl-dev` must be present at build time.
+`openssl@3` can be installed on macOS, among others, through `brew`.
 
 ## Development Setup