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Using consistent cluster management and not using schema commitlog ends with a bad configuration throw during bootstrap. Soon, we will make consistent cluster management mandatory. This forces us to also make schema commitlog mandatory, which we do in this patch. A booting node decides to use schema commitlog if at least one of the two statements below is true: - the node has `force_schema_commitlog=true` config, - the node knows that the cluster supports the `SCHEMA_COMMITLOG` cluster feature. The `SCHEMA_COMMITLOG` cluster feature has been added in version 5.1. This patch is supposed to be a part of version 6.0. We don't support a direct upgrade from 5.1 to 6.0 because it skips two versions - 5.2 and 5.4. So, in a supported upgrade we can assume that the version which we upgrade from has schema commitlog. This means that we don't need to check the `SCHEMA_COMMITLOG` feature during an upgrade. The reasoning above also applies to Scylla Enterprise. Version 2024.2 will be based on 6.0. Probably, we will only support an upgrade to 2024.2 from 2024.1, which is based on 5.4. But even if we support an upgrade from 2023.x, this patch won't break anything because 2023.1 is based on 5.2, which has schema commitlog. Upgrades from 2022.x definitely won't be supported. When we populate a new cluster, we can use the `force_schema_commitlog=true` config to use schema commitlog unconditionally. Then, the cluster feature check is irrelevant. This check could fail because we initiate schema commitlog before we learn about the features. The `force_schema_commitlog=true` config is especially useful when we want to use consistent cluster management. Failing feature checks would lead to crashes during initial bootstraps. Moreover, there is no point in creating a new cluster with `consistent_cluster_management=true` and `force_schema_commitlog=false`. It would just cause some initial bootstraps to fail, and after successful restarts, the result would be the same as if we used `force_schema_commitlog=true` from the start. In conclusion, we can unconditionally use schema commitlog without any checks in 6.0 because we can always safely upgrade a cluster and start a new cluster. Apart from making schema commitlog mandatory, this patch adds two changes that are its consequences: - making the unneeded `force_schema_commitlog` config unused, - deprecating the `SCHEMA_COMMITLOG` feature, which is always assumed to be true. Closes scylladb/scylladb#16254