fix: Add quick information about userdev 1.21.4 and beta versions (#513)

Co-authored-by: Matouš Kučera <mk@kcra.me>

Author: Strokkur424

docs/paper/dev/getting-started/userdev.mdx

@@ -49,6 +49,16 @@ plugins {
 The latest version of `paperweight-userdev` supports dev bundles for Minecraft 1.17.1 and newer, so it's best practice to keep it up to date!
 Only the latest version of `paperweight-userdev` is officially supported, and we will ask you to update first if you are having problems with old versions.
 
+:::warning
+
+In order to use userdev for 1.21.4, you have to update your userdev to a beta version. At the time of writing, the latest version that works for
+1.21.4 is `2.0.0-beta.8`. You can find the latest version here: https://github.com/PaperMC/paperweight/releases.
+
+Furthermore, from version `2.0.0` onwards, userdev requires you to use at least **Gradle 8.11.2** (Gradle 8.11 or below will not work). For more information on upgrading, check
+out this site from the Gradle docs: https://docs.gradle.org/current/userguide/gradle_wrapper.html.
+
+:::
+
 :::info[Snapshots]
 
 **paperweight-userdev** SNAPSHOT (pre-release) versions are only available through Paper's Maven repository.