Fix broken gradle repo URL

[steventrouble]

/gradle/ doesn't work anymore, but /artifactory/ works.

The PR is in accordance with the Developer's Certificate of Origin.

---

Tested: ./gradlew build && ./gradlew test

Before:

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':node-driver:compileKotlin'.
> Could not resolve all files for configuration ':node-driver:compileClasspath'.
   > Could not find org.gradle:gradle-tooling-api:5.6.4.

After:

net.corda.core.crypto.CryptoUtilsTest > test default SecureRandom uses platformSecureRandom FAILED
210 tests completed, 1 failed, 3 skipped

[nargas-ritu]

Internal ticket CORDA-4187

[marcellofarina75]

Hello steventrouble,
thank you for your contribution to Corda!

Our engineers reviewed your Pull Request and established that, while your proposal would definitely improve Corda, the work needed to merge it (in terms of unit, integration and regression tests, documentation updates, security assessments, and ongoing maintenance) would prevent us from shipping features and fixes that are prioritised higher in our roadmap (which is driven directly from inputs from across the user base).

So this means we regretfully need to let you know that your PR will be declined.

Please feel free to contact devrel@r3.com if you need any further information