Rename master/slave in Systems.md

[darylldoyle]

Renamed master/slave terminology to primary/replica.

Description of the Change

This PR changes the master/slave terminology to use primary/replica when talking about databases within System.md. This PR will close https://github.com/10up/Engineering-Best-Practices/issues/339.

Benefits

Brings the best practice document inline with 10up's policy on naming conventions.

Possible Drawbacks

None that I can see.

Checklist:

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

Applicable Issues

Changelog Entry

## Changed
- Database relationship naming from `master` to `primary` and from `slave` to `replica`

[jeffpaul]

Confirmed with @TheLastCicada that this is good to merge, will click the Big Green Button™️ momentarily.

[jeffpaul]

Nice catch and thanks for the PR @darylldoyle!