Update adventure to v4.21.0 (#4642)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

gradle/libs.versions.toml

@@ -7,7 +7,7 @@ checkerqual = "3.49.2"
 gson = "2.10"
 guava = "31.1-jre"
 snakeyaml = "2.0"
-adventure = "4.20.0"
+adventure = "4.21.0"
 adventure-bukkit = "4.3.4"
 log4j = "2.19.0"