Use targetRefs and sourceRefs instead of refs in pre-translations

[pmachapman]

This PR removes support for the old refs property for pre-translations, and instead supports sourceRefs and targetRefs.

Partial fix for sillsdev/serval#940.

---

This change is 

[Enkidu93]

@Enkidu93 reviewed 6 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on ddaspit).

[ddaspit]

@ddaspit reviewed 6 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on pmachapman).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.53%. Comparing base (29d4dbb) to head (72b3d37).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #297   +/-   ##
=======================================
  Coverage   91.53%   91.53%           
=======================================
  Files         355      355           
  Lines       23026    23028    +2     
=======================================
+ Hits        21077    21079    +2     
  Misses       1949     1949           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.