Bump io.mockk:mockk from 1.13.4 to 1.13.8 in /http4k-app

Bumps [io.mockk:mockk](https://github.com/mockk/mockk) from 1.13.4 to 1.13.8.
- [Release notes](https://github.com/mockk/mockk/releases)
- [Commits](mockk/mockk@v1.13.4...1.13.8)

---
updated-dependencies:
- dependency-name: io.mockk:mockk
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

http4k-app/build.gradle

@@ -64,6 +64,6 @@ dependencies {
     testImplementation "org.junit.jupiter:junit-jupiter-api:${junitVersion}"
     testImplementation "org.junit.jupiter:junit-jupiter-engine:${junitVersion}"
 
-    testImplementation "io.mockk:mockk:1.13.4"
+    testImplementation "io.mockk:mockk:1.13.8"
     testImplementation "org.testcontainers:junit-jupiter:1.19.1"
 }