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.

server/build_without_gradle.sh

@@ -17,7 +17,7 @@ BUILD_TOOLS=${ANDROID_BUILD_TOOLS:-29.0.2}
 BUILD_DIR="$(realpath ${BUILD_DIR:-build_manual})"
 CLASSES_DIR="$BUILD_DIR/classes"
 SERVER_DIR=$(dirname "$0")
-SERVER_BINARY=scrcpy-server.jar
+SERVER_BINARY=scrcpy-server
 
 echo "Platform: android-$PLATFORM"
 echo "Build-tools: $BUILD_TOOLS"
@@ -59,4 +59,4 @@ cd "$BUILD_DIR"
 jar cvf "$SERVER_BINARY" classes.dex
 rm -rf classes.dex classes
 
-echo "Server generated in $BUILD_DIR/scrcpy-server.jar"
+echo "Server generated in $BUILD_DIR/$SERVER_BINARY"