Upgraded kotlin.version from 1.3.11 to 1.3.21 (#173)

Bumps `kotlin.version` from 1.3.11 to 1.3.21.

Updates `kotlin-compiler-embeddable` from 1.3.11 to 1.3.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.3.11...v1.3.21)

Updates `kotlin-reflect` from 1.3.11 to 1.3.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.3.11...v1.3.21)

Updates `kotlin-stdlib` from 1.3.11 to 1.3.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.3.11...v1.3.21)

Updates `kotlin-stdlib-jdk8` from 1.3.11 to 1.3.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.3.11...v1.3.21)

Updates `kotlin-test` from 1.3.11 to 1.3.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.3.11...v1.3.21)

Updates `kotlin-test-junit` from 1.3.11 to 1.3.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.3.11...v1.3.21)

Updates `kotlin-maven-plugin` from 1.3.11 to 1.3.21

Signed-off-by: dependabot[bot] <support@dependabot.com>

pom.xml

@@ -42,7 +42,7 @@
 
   <properties>
     <doxia.version>1.8</doxia.version>
-    <kotlin.version>1.3.11</kotlin.version>
+    <kotlin.version>1.3.21</kotlin.version>
     <ktlint.version>0.29.0</ktlint.version>
     <maven.compiler.source>1.8</maven.compiler.source>
     <maven.compiler.target>1.8</maven.compiler.target>