Add 1.21.4 for bug reports (#3106)

Author: TheMeinerLP

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -27,6 +27,7 @@ body:
       description: Which server version version you using? If your server version is not listed, it is not supported. Update to a supported version first.
       multiple: false
       options:
+        - '1.21.4'
         - '1.21.3'
         - '1.21.1'
         - '1.20.6'