ResourceFieldEnhancerBase triggers return type deprecations on Drupal 12

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Problem/Motivation

On Drupal 12, every test that loads a class extending ResourceFieldEnhancerBase gets 3 deprecations from Symfony's DebugClassLoader:

Method "Shaper\DataAdaptor\ReversibleTransformationValidationInterface::getInternalValidator()" might add "ValidateableInterface" as a native return type declaration in the future. Do the same in implementation "Drupal\jsonapi_extras\Plugin\ResourceFieldEnhancerBase" now to avoid errors or add an explicit @return annotation to suppress this message.

The same message comes for getInputValidator() and getOutputValidator().

run-tests.sh exits 1 for a test class with deprecations, even when all tests pass. So modules with an enhancer plugin get a failing "PHPUnit (next major)" job. Geofield is one: see #3598572: Automated Drupal 12 compatibility fixes for geofield 10.3.x-dev.

Steps to reproduce

On Drupal 12.0.0-beta2 with PHP 8.5, run Geofield's GeofieldGeoJsonEnhancerTest with !50 of #3598572: Automated Drupal 12 compatibility fixes for geofield 10.3.x-dev: "Tests: 15, Assertions: 28, Deprecations: 3."

Proposed resolution

Add @return \Shaper\Validator\ValidateableInterface to the three methods. The message itself suggests this. With it, the same test passes with no deprecations.

A native return type would also work, but it would break enhancers in other modules that override these methods without a return type.

Remaining tasks

• Merge request.
• Review.

User interface changes

None.

API changes

None. Only docblocks change.

Data model changes

None.