Save the version in VERSION.txt instead of VERSION

Fixes https://github.com/libsdl-org/SDL/issues/6558

build-scripts/showrev.sh

@@ -5,8 +5,8 @@
 SDL_ROOT=$(dirname $0)/..
 cd $SDL_ROOT
 
-if [ -e ./VERSION ]; then
-    cat ./VERSION
+if [ -e ./VERSION.txt ]; then
+    cat ./VERSION.txt
     exit 0
 fi