Updates for Minecraft 1.21 (#4) To satisfy the Java version requirement changes in Minecraft 1.21, the steps about `setup-java` is changed. - the distribution is changed to "temurin" from "adopt", as the old one is depreacted and moved to the new one. - the version is changed to "21" from "17", as MC required. And there are some other document and workflow script changes. - Updated README. - Adds the trigger on push, so that it will be invoked on PR commit.