fix: Remove jfrog repositories

[jan-goral]

JFrog repository becomes unavailable, dependencies cannot be resolved which is blocking daily work.
Typically Gradle build is stuck on task:

> Resolve dependencies of :test_runner:runtimeClasspath

also is throwing:

Could not resolve all dependencies for configuration ':test_runner:runtimeClasspath'.
   > Could not resolve com.google.http-client:google-http-client-appengine:1.39.2.
     Required by:
         project :test_runner > com.google.cloud:google-cloud-storage:1.117.1
      > Could not resolve com.google.http-client:google-http-client-appengine:1.39.2.
         > Could not get resource 'https://dl.bintray.com/kotlin/ktor/com/google/http-client/google-http-client-appengine/1.39.2/google-http-client-appengine-1.39.2.pom'.
            > Could not HEAD 'https://dl.bintray.com/kotlin/ktor/com/google/http-client/google-http-client-appengine/1.39.2/google-http-client-appengine-1.39.2.pom'. Received status code 502 from server: Bad Gateway

Looks like all flank dependencies are available on the mavenCentral, so we can depend on it.

[github-actions]

Timestamp: 2021-07-20 08:25:02
Buildscan url for ubuntu-workflow run 1048126084
https://gradle.com/s/bu4pfmmcunxh2

[Sloox]

👍

[Sloox]

@flank-it

[github-actions]

Integration tests succeed for all OSs ✅
Windows Build scan:
MacOS Build scan: https://gradle.com/s/c3dniefqkbnl6
Linux Build scan: https://gradle.com/s/swxumi427rgcq
Workflow run https://github.com/Flank/flank/actions/runs/1048070329