re-activate auto-download of JDK with Gradle Toolchain support (#7)

Co-authored-by: Michael Hoennig <michael@hoennig.de>
Reviewed-on: https://dev.hostsharing.net/hostsharing/hs.hsadmin.ng/pulls/7
Reviewed-by: Timotheus Pokorra <timotheus.pokorra@hostsharing.net>

settings.gradle

@@ -7,6 +7,10 @@ pluginManagement {
     }
 }
 
+plugins {
+    id 'org.gradle.toolchains.foojay-resolver-convention' version '0.7.0'
+}
+
 dependencyResolutionManagement {
     components {
         all {