chore(gradle): bump org.http4k:http4k-bom in /mock-api

Bumps [org.http4k:http4k-bom](https://github.com/http4k/http4k) from 5.13.8.0 to 5.13.9.0.
- [Release notes](https://github.com/http4k/http4k/releases)
- [Changelog](https://github.com/http4k/http4k/blob/master/CHANGELOG.md)
- [Commits](http4k/http4k@5.13.8.0...5.13.9.0)

---
updated-dependencies:
- dependency-name: org.http4k:http4k-bom
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

mock-api/build.gradle.kts

@@ -9,7 +9,7 @@ repositories {
 }
 
 dependencies {
-    implementation(platform("org.http4k:http4k-bom:5.13.8.0"))
+    implementation(platform("org.http4k:http4k-bom:5.13.9.0"))
     implementation("org.http4k:http4k-core")
     implementation("org.http4k:http4k-client-apache")