🛠️ [TASK] : Bump Scylla Library to 0.15

[stevenj]

A problem in the Scylla Driver was causing us issues with linting. This caused us to have to silence The errors with a weird code construction. This was fixed by @Mr-Leshiy in PR: scylladb/scylla-rust-driver#1077 and was released in version 0.15 of the driver.

Bump the version of the crate to 0.15, and remove the hacky module structure required to silence the lint which is no longer required.

Search for: TODO: https://github.com/input-output-hk/catalyst-voices/issues/828 to find all the places that require fixing.

Make sure everything builds, and indexing still works properly.
Closes: #828