Deprecate the concurrently-init-stats configuration

[Rustin170506]

Enhancement

This configuration controls whether stats are initialized concurrently in non-lite init stats mode.

The current default value is true, as we believe it makes sense to initialize stats concurrently by default. The reasons are as follows:

1. It only runs once when the TiDB node starts.
2. For clusters with a large number of tables, single-threaded stats initialization is too slow. See Init stats failed: GC life time is shorter than transaction duration #60138
3. There is no need to expose this implementation detail to users anymore.

So the proposal is to deprecate this configuration and initialize stats concurrently by default.