Summary:
With 4aabc0ccc8673eef1314d3d896a518289827f60e/D36298 we dynamically get the min replicated index from the cdc_state table. We no longer need to set `set_cdc_min_replicated_index` during xCluster checkpoint since the cdc_service on all peers will hold up the WAL within `FLAGS_xcluster_checkpoint_max_staleness_secs`, which is guaranteed to be less than `FLAGS_log_min_seconds_to_retain`.

Fixes #24105
Jira: DB-12994

Test Plan: Jenkins

Reviewers: yyan, xCluster

Reviewed By: yyan

Subscribers: ycdcxcluster, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D38347