build(deps): bump kotlin_version from 1.7.10 to 1.7.21 in /android

Bumps `kotlin_version` from 1.7.10 to 1.7.21.

Updates `kotlin-gradle-plugin` from 1.7.10 to 1.7.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](https://github.com/JetBrains/kotlin/commits)

Updates `kotlin-stdlib-jdk7` from 1.7.10 to 1.7.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](https://github.com/JetBrains/kotlin/commits)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-gradle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin:kotlin-stdlib-jdk7
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

android/build.gradle

@@ -2,7 +2,7 @@ group 'dev.steenbakker.nordicdfu'
 version '1.0-SNAPSHOT'
 
 buildscript {
-    ext.kotlin_version = '1.7.10'
+    ext.kotlin_version = '1.7.21'
     repositories {
         google()
         mavenCentral()