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/gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-7.6.1-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-7.5-all.zip