ddl-introduction: Remove pre-v6.2 info

[dveeden]

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[qiancai]

The Execution Principles and Best Practices document was added in v6.2 and later versions only to explain the parallel DDL framework. In this case, we could consider simply switching the order of the ‘div’ elements so that users upgrading TiDB clusters can more easily understand the differences. @wjhuang2016 PTAL

[wjhuang2016]

LGTM