chore: Enable legacy libraries migration by default [FC-0107]

[ChrisChV]

Description

• Deletes the ENABLE_LEGACY_LIBRARY_MIGRATOR and enable the legacy libraries migration by default
• Which user roles will this change impact? "Course Author"

Testing instructions

Verify that you don't need to set ENABLE_LEGACY_LIBRARY_MIGRATOR to enable the legacy migration flow

Other information

Merge before Ulmo Cut

Best Practices Checklist

We're trying to move away from some deprecated patterns in this codebase. Please
check if your PR meets these recommendations before asking for a review:

• Any new files are using TypeScript (.ts, .tsx).
• Deprecated propTypes, defaultProps, and injectIntl patterns are not used in any new or modified code.
• Tests should use the helpers in src/testUtils.tsx (specifically initializeMocks)
• Do not add new fields to the Redux state/store. Use React Context to share state among multiple components.
• Use React Query to load data from REST APIs. See any apiHooks.ts in this repo for examples.
• All new i18n messages in messages.ts files have a description for translators to use.
• Imports avoid using ../. To import from parent folders, use @src, e.g. import { initializeMocks } from '@src/testUtils'; instead of from '../../../../testUtils'

[openedx-webhooks]

Thanks for the pull request, @ChrisChV!

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.74%. Comparing base (5ce61fa) to head (525bd79).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2561      +/-   ##
==========================================
- Coverage   94.74%   94.74%   -0.01%     
==========================================
  Files        1218     1218              
  Lines       27299    27298       -1     
  Branches     5983     6144     +161     
==========================================
- Hits        25865    25864       -1     
+ Misses       1376     1363      -13     
- Partials       58       71      +13     

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[bradenmacdonald]

👍

• I tested this: made sure I didn't have the setting, and that the migrator appeared
• I read through the code
• I checked for accessibility issues: n/a
• Includes documentation: n/a