Update .idea/ and README to use JDK 17 instead of 15

Fixes #126.

.idea/misc.xml

 <?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
   <component name="ExternalStorageConfigurationManager" enabled="true" />
-  <component name="ProjectRootManager" version="2" languageLevel="JDK_1_8" project-jdk-name="azul-15" project-jdk-type="JavaSDK">
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_1_8" project-jdk-name="openjdk-17" project-jdk-type="JavaSDK">
     <output url="file://$PROJECT_DIR$/out" />
   </component>
 </project>
\ No newline at end of file

README.md

@@ -82,13 +82,15 @@ To specify a different data directory and enable the debug output:
 
     ./gradlew run --args="--debug --data-dir=/tmp/briar-tmp"
 
-In case you want to build a self-contained Debian package, note that you need at least JDK 15 and that you might need to install `binutils` and `fakeroot` before running the respective task:
+In case you want to build a self-contained Debian package, note that you need at least JDK 17 and that you might need to install `binutils` and `fakeroot` before running the respective task:
 
     ./gradlew packageDeb
 
 If you want to use the JDKs downloaded by Intellij IDEA, you can for example specify the path like this:
 
-    ./gradlew packageDeb -Dorg.gradle.java.home=/home/briar/.jdks/azul-15.0.4
+    ./gradlew packageDeb -Dorg.gradle.java.home=$HOME/.jdks/openjdk-17.0.2
+
+Take a look at [_.gitlab-ci.yml_](.gitlab-ci.yml) if you have problems with Kotlin and JDK 17.
 
 ## Translations