Fix @return doc of LanguageTestTrait::createLanguageFromLangcode [#3636644]

Problem/Motivation

\Drupal\Tests\language\Traits\LanguageTestTrait::createLanguageFromLangcode documents the return type as \Drupal\Core\Language\ConfigurableLanguageInterface but it should be \Drupal\language\ConfigurableLanguageInterface. The native return type is correct. Since phpstan is configured to treat phpdoc types as certain it means we have 23 violations of this in the baseline.

Steps to reproduce

git grep 'ConfigurableLanguageInterface is not subtype' -- core/.phpstan-baseline.php

Proposed resolution

Fix the @return doc or remove it

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet

Issue fork drupal-3636644

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3636644-language-test-trait changes, plain diff MR !17748
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