fix(deps): update kotlin monorepo to v2.1.21 (#33)

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

Author: renovate[bot]

gradle/libs.versions.toml

@@ -1,5 +1,5 @@
 [versions]
-kotlin = "2.1.20"
+kotlin = "2.1.21"
 coroutines = "1.10.2"
 gradle-ktlint = "12.2.0"
 antlr = "4.13.2"