Add support for Postgres 13.x

[marceloboeira]

Description

As mentioned here https://github.com/checkr/flagr/issues/466 Postgres 9.6 is deprecated and no longer receiving security patches, therefore, we need to start supporting other versions.

For now, we keep testing for pgsql9.6 but also testing for the latest available version (for production), which seems to be 13.x

Thanks @jnikitin-r7 for the issue

How Has This Been Tested?

Adding mainly integration testing with different pgsql versions

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[marceloboeira]

cc @jnikitin-r7