Bump kotlin-gradle-plugin from 1.5.20 to 1.5.21 (#663)

Bumps kotlin-gradle-plugin from 1.5.20 to 1.5.21.

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-gradle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

build.gradle

@@ -21,7 +21,7 @@ import org.springframework.boot.gradle.plugin.SpringBootPlugin
 
 buildscript {
     ext {
-        kotlin_version = '1.5.20'
+        kotlin_version = '1.5.21'
         spring_boot_version = '2.3.5.RELEASE'
     }