Bump kotlin-gradle-plugin from 1.6.21 to 1.7.21 (#298)

Bumps [kotlin-gradle-plugin](https://github.com/JetBrains/kotlin) from 1.6.21 to 1.7.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.6.21...v1.7.21)

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

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Sergio del Amo <[email protected]>

buildSrc/build.gradle

@@ -9,6 +9,6 @@ repositories {
 
 dependencies {
     implementation 'io.micronaut.gradle:micronaut-gradle-plugin:3.6.5'
-    implementation("org.jetbrains.kotlin:kotlin-gradle-plugin:1.6.21")
+    implementation("org.jetbrains.kotlin:kotlin-gradle-plugin:1.7.21")
     implementation("org.jetbrains.kotlin:kotlin-allopen:1.7.21")
 }