CassandraConnection: Replace generic codec with direct usage of CassandraCodec

[rukai]

There is no advantage to using generics here as CassandraConnection will only ever be used with a CassandraCodec so we should directly use CassandraCodec to improve code readability.

[conorbros]

lol, pretty sure this was me. Good catch.

[github-actions]

1 benchmark regressed. 0 benchmark improved. Please check the benchmark workflow logs for full details: https://github.com/shotover/shotover-proxy/actions/runs/2945602356

Found 4 outliers among 100 measurements (4.00%)
  3 (3.00%) high mild
  1 (1.00%) high severe
transform/redis_timestamp_tagger_tagged
                        time:   [1.9117 us 1.9400 us 1.9679 us]
                        change: [+22.926% +25.648% +28.354%] (p = 0.00 < 0.05)
                        Performance has regressed.