fix(deps): update kotlin monorepo to v2.1.21 (#265)

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

Author: renovate[bot]

gradle/libs.versions.toml

@@ -6,7 +6,7 @@ micronaut-logging = "1.6.0"
 micronaut-serde = "2.14.0"
 micronaut-test = "4.7.0"
 groovy = "4.0.18"
-kotlin = '2.1.20'
+kotlin = '2.1.21'
 sonatype-scan = "3.0.0"
 
 [libraries]