The compatibility level for mixins in master and all backports is Java 17. However, according to Minecraft Wiki, the minimum Java version for 1.12 - 1.16.5 is Java 8, 1.17 - 1.17.1 requires Java 16, 1.18 - 1.20.4 requires Java 17, and 1.20.5+ requires Java 21. So, it may be necessary to set the compatibility level for the 1.16 backport to Java 8, and possibly set master to Java 21.
Additionally, the build.gradle specifies the target Java version as 17 for the 1.16 backport. This may need to be changed as well.
The compatibility level for mixins in master and all backports is Java 17. However, according to Minecraft Wiki, the minimum Java version for 1.12 - 1.16.5 is Java 8, 1.17 - 1.17.1 requires Java 16, 1.18 - 1.20.4 requires Java 17, and 1.20.5+ requires Java 21. So, it may be necessary to set the compatibility level for the 1.16 backport to Java 8, and possibly set master to Java 21. Additionally, the build.gradle specifies the target Java version as 17 for the 1.16 backport. This may need to be changed as well.