[HttpClient] Improve error when no client implementation found (#678)

Add the:
 `3. Using JDK 23+, and need to set property <maven.compiler.proc>full</maven.compiler.proc>`

... just to be a bit more explicit. The `avaje-http-client-generator dependency was not available` is accurate but perhaps it's good to explicitly mention the maven.compiler.proc full issue.

Co-authored-by: Rob Bygrave <robin.bygrave@gmail.com>

Author: rob-bygrave

http-client/src/main/java/io/avaje/http/client/DHttpApi.java

@@ -42,7 +42,8 @@ <T> T provideFor(Class<T> type, HttpClient httpClient) {
               + type
               + "\nPossible Causes: \n"
               + "1. Missing @Client or @Client.Import annotation.\n"
-              + "2. The avaje-http-client-generator dependency was not available during compilation\n");
+              + "2. The avaje-http-client-generator dependency was not available during compilation\n"
+              + "3. Using JDK 23+, and need to set property <maven.compiler.proc>full</maven.compiler.proc>\n");
     }
     return apiProvider.provide(httpClient);
   }