Add a note about backporting columns with default

What does this MR do?

Add a note about backporting columns with default

PostgreSQL 11 is required starting from 12.10. Before PostgreSQL 11, adding a column requires a full table write, so if the table has a considerable number of records, this causes for the table to be unavailable until the column is added (which can take a while).

This commits adds a note on 'Add Column with Default' to state backporting a column with default needs to use add_column_with_default, and if a large table is involved, it should not add a default

Related issues

Related to #207820 (closed) and to !32375 (comment 344716016)

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