chore: rollback gradlup shadow plugin to version 8 (#144)

version 9 of the plugin requires a minimum Java version 11

Signed-off-by: Vladimir Petko <vladimir.petko@canonical.com>

Author: vpa1977

rockcraft-gradle/build.gradle.kts

@@ -1,5 +1,5 @@
 plugins {
-    id("com.gradleup.shadow") version "9.0.1"
+    id("com.gradleup.shadow") version "8.3.8"
     `java-gradle-plugin`
     id("com.gradle.plugin-publish") version "1.3.1"
     id ("org.gradlex.reproducible-builds") version "1.0"