[15:00:45] [main/FATAL] [Sponge]: //
[15:00:45] [main/FATAL] [Sponge]: / Old Mixin Version Loaded! /
[15:00:45] [main/FATAL] [Sponge]: //
[15:00:45] [main/FATAL] [Sponge]: /* Hey, sorry, but Sponge requires a newer version of Mixin being loaded, and unfortunately /
[15:00:45] [main/FATAL] [Sponge]: / with an older version, nothing will work as it should. Please rename the sponge jar to load /
[15:00:45] [main/FATAL] [Sponge]: / earlier than other coremods, so that Sponge's Mixin version will be loaded (they're backwards /
[15:00:45] [main/FATAL] [Sponge]: / compatible, but not forwards compatible). We suggest doing this by prefixing the Sponge jar /
[15:00:45] [main/FATAL] [Sponge]: / filename with "aaa". We're sorry for the inconvenience, but this is all that we can do. /
[15:00:45] [main/FATAL] [Sponge]: / /
[15:00:45] [main/FATAL] [Sponge]: / Current Loaded Mixin : 0.7.11 /
[15:00:45] [main/FATAL] [Sponge]: / Required Mixin Version : 0.8 /
[15:00:45] [main/FATAL] [Sponge]: /************************************************************************************************/

Try the prefixing solution the log gives. Prefix the sponge jar.

Mixin 0.8 only works on 1.14 and above

Try the prefixing solution the log gives. Prefix the sponge jar.

This solution helps, but why not update?