Nightly Spigot 2024-08-18

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.