Fix Gradle project sync error

[PhilLab]

The commit 5fd2e29 introduced stricter dependency verification, but this resulted in the Gradle project syncing to fail with the error:

Multiple build operations failed.
Dependency verification failed for configuration ':app:androidTestUtil':

• On artifact gradle-9.2.1-src.zip (gradle:gradle:9.2.1) in repository
  'Gradle distributions': Artifact was signed with key 'E2F38302C8075E3D'
  but it wasn't found in any key server so it couldn't be verified
  This can indicate that a dependency has been compromised. Please
  carefully verify the signatures and checksums

This happened independent of the used gradle version..

The followup 32545d4 added the key's fingerprint, but this was not enough to establish trust. Adding the public key to the keyring. The key was taken from
https://github.com/gradle/gradle/blob/b99fe7d304e68b978011840c31c08068f671f794/KEYS.md
and cross-checked with https://gradle.org/keys/

• Tests written, or not not needed

[PhilLab]

@alperozturk96 I am unsure why this didn't cause issues for more developers but at least for me, building was impossible on two different machines

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