sqlfluff: Upgrading to 3.x

[UN-9BOT]

https://docs.sqlfluff.com/en/stable/releasenotes.html#upgrading-to-3-x

The serialised output for sqlfluff lint now contains more information about the span of linting issues and initial proposed fixes. Beside the new fields, the original fields of line_pos and line_no have been renamed to start_line_pos and start_lineno, to distinguish them from the new fields starting end*.

[mfussenegger]

Thanks