[IS-7 Docs] Remove the "Change the Default Datasource for BPS" documentation from IS 7 docs since workflow feature was not availble in IS pack by default

[UdeshAthukorala]

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With this improvement[1], we have removed the workflow feature from the default IS pack, and the relevant implementations have been moved to a separate connector.

However, the IS 7.0 documentation still includes the section "Change the Default Datasource for BPS,"[2] which is part of the workflow feature. Therefore, we need to remove this documentation from the IS 7.0 guide and move it to the workflow connector documentation[3].

[1] https://github.com/wso2/product-is/issues/17789 [2] https://is.docs.wso2.com/en/latest/deploy/configure/databases/carbon-database/change-datasource-bpsds/ [3]https://github.com/wso2-extensions/identity-workflow-impl-bps/blob/master/docs/config.md