tidb: Remove warning about experimental feature for collecting predicate columns and update default value for tidb_enable_collect_predicate_columns

[Rustin170506]

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[Oreoxmt]

Suggested change

	- Default value: `ON`
	- Default value: `ON`. Before v8.3.0, the default value is `OFF`.

[Oreoxmt]

Please update release-8.3.0.md:

docs/releases/release-8.3.0.md

Lines 209 to 214 in ff96e35

	### System variables
	| Variable name | Change type | Description |
	|--------|------------------------------|------|
	| [`tidb_ddl_reorg_batch_size`](/system-variables.md#tidb_ddl_reorg_batch_size) | Modified | Adds the SESSION scope. |
	| [`tidb_ddl_reorg_worker_cnt`](/system-variables.md#tidb_ddl_reorg_worker_cnt) | Modified | Adds the SESSION scope. |

| [`tidb_enable_column_tracking`](/system-variables.md#tidb_enable_column_tracking-new-in-v540) | Modified | Changes the default value from `OFF` to `ON` after further tests, which means that TiDB collects `PREDICATE COLUMNS` by default. |