Rename scrcpy-server.jar to scrcpy-server

The server name ending with .jar has several drawbacks:
 - meson requires the jar executable to attempt to modify it:
     <https://github.com/Genymobile/scrcpy/issues/404#issuecomment-456065923>
     <https://github.com/mesonbuild/meson/issues/4844>
 - meson warns during "ninja install"
     <https://github.com/Genymobile/scrcpy/issues/458>
 - some users try to execute it on the computer as a java executable

Removing the extension solves all these problems.

app/src/server.c

@@ -13,7 +13,7 @@
 #include "net.h"
 
 #define SOCKET_NAME "scrcpy"
-#define SERVER_FILENAME "scrcpy-server.jar"
+#define SERVER_FILENAME "scrcpy-server"
 
 #define DEFAULT_SERVER_PATH PREFIX "/share/scrcpy/" SERVER_FILENAME
 #define DEVICE_SERVER_PATH "/data/local/tmp/" SERVER_FILENAME
@@ -32,7 +32,7 @@ get_server_path(void) {
     // the absolute path is hardcoded
     return DEFAULT_SERVER_PATH;
 #else
-    // use scrcpy-server.jar in the same directory as the executable
+    // use scrcpy-server in the same directory as the executable
     char *executable_path = get_executable_path();
     if (!executable_path) {
         LOGE("Could not get executable path, "