fix: keep useFactory root providers for kept injectables (#7938)

[satanTime]

Closes #7938

Summary

• preserve kept root injectables that are backed by useFactory metadata
• add a regression for NonNullableFormBuilder kept alongside ReactiveFormsModule
• document the explicit keep requirement for NonNullableFormBuilder in the FAQ
• keep the regression Angular-10-safe by resolving NonNullableFormBuilder at runtime

Validation

• COMPOSE_PROJECT_NAME=ngmocks_7938_1774684993 sh test.sh root
• COMPOSE_PROJECT_NAME=ngmocks_7938_1774684993 sh test.sh a5es5
• COMPOSE_PROJECT_NAME=ngmocks_7938_1774684993 sh test.sh a10
• COMPOSE_PROJECT_NAME=ngmocks_7938_1774684993 sh test.sh a14
• COMPOSE_PROJECT_NAME=ngmocks_7938_1774684993 sh test.sh a20
• COMPOSE_PROJECT_NAME=ngmocks_7938_1774684993 sh test.sh coverage

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (46e4423) to head (1c909c0).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main    #13468   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          228       229    +1     
  Lines         4168      4175    +7     
  Branches      1187      1192    +5     
=========================================
+ Hits          4168      4175    +7     

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