Upgrade to 1.21.5

Changelog

+ Update missing token and channel ID error messages.
+ Upgrade JDA version from '5.0.0-beta.24' to '5.5.0'.
+ Upgrade Spigot from '1.21.4-R0.1-SNAPSHOT' to '1.21.5-R0.1-SNAPSHOT'.

Full Changelog: v1.3.0...v1.3.1