Fix flutter project with new android-studio version:

- upgrade gradle from 7.5 to 7.6.1
  - change java target from 1.8 to 17
  - set flutter jdk-dir to system-jdk (instead of now-incompatible android-studio bundled jdk)
    (android studio bundles jdk 21, jdk 21 needs gradle 8.4, flutetr does not work with gradle > 7.6.1 )
    => see https://docs.gradle.org/current/userguide/compatibility.html#java
    =>     https://www.liquidbcn.com/en/insights/solving-issues-android-studio-ladybug-java-21-flutter

flutter/android/app/build.gradle

@@ -38,12 +38,12 @@ android {
 
     compileOptions {
         coreLibraryDesugaringEnabled true
-        sourceCompatibility JavaVersion.VERSION_1_8
-        targetCompatibility JavaVersion.VERSION_1_8
+        sourceCompatibility JavaVersion.VERSION_17
+        targetCompatibility JavaVersion.VERSION_17
     }
 
     kotlinOptions {
-        jvmTarget = '1.8'
+        jvmTarget = '17'
     }
 
     sourceSets {