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>

build.gradle

@@ -43,6 +43,8 @@ repositories {
 java {
     toolchain {
         languageVersion = JavaLanguageVersion.of(21)
+        vendor = JvmVendorSpec.ADOPTIUM
+        implementation = JvmImplementation.VENDOR_SPECIFIC
     }
 }