GUACAMOLE-1859: Update to Kotlin 1.9.21.

doc/licenses/kotlin-1.9.21/NOTICE.txt

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+Kotlin Compiler
+Copyright 2010-2023 JetBrains s.r.o and respective authors and developers

doc/licenses/kotlin-1.9.21/README

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+Kotlin (https://kotlinlang.org/)
+--------------------------------
+
+    Version: 1.9.21
+    From: 'JetBrains s.r.o and respective authors and developers'
+    License(s):
+        Apache v2.0
+

doc/licenses/kotlin-1.9.21/dep-coordinates.txt

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+org.jetbrains.kotlin:kotlin-reflect:jar:1.9.21
+org.jetbrains.kotlin:kotlin-stdlib:jar:1.9.21
+org.jetbrains.kotlin:kotlin-stdlib-common:jar:1.9.21
+org.jetbrains.kotlin:kotlin-stdlib-jdk8:jar:1.9.21
+org.jetbrains.kotlin:kotlin-stdlib-jdk7:jar:1.9.21