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.

BUILD.md

@@ -225,7 +225,7 @@ sudo ninja install    # without sudo on Windows
 This installs two files:
 
  - `/usr/local/bin/scrcpy`
- - `/usr/local/share/scrcpy/scrcpy-server.jar`
+ - `/usr/local/share/scrcpy/scrcpy-server`
 
 Just remove them to "uninstall" the application.
 
@@ -244,7 +244,7 @@ configuration:
 
 ```bash
 meson x --buildtype release --strip -Db_lto=true \
-    -Dprebuilt_server=/path/to/scrcpy-server.jar
+    -Dprebuilt_server=/path/to/scrcpy-server
 cd x
 ninja
 sudo ninja install