Issue #3589046: DomainConfigOverrideMigration: drop the langcode-shape regex, use installed-languages list as the only signal

Frank Mablyrequested to merge
issue/domain-3589046:3589046-domainconfigoverridemigration-drop-the into 3.x

Linked issue: https://www.drupal.org/project/domain/issues/3589046

See commit message for full rationale and the parallel 3.0.x change in MR !380 / #3589035.

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