Feature: Gitlab CI definition.

Lifted in its entirety from the Drupal Association .gitlab-ci.yml template.

Also, updated and pruned dev dependencies for PHP 8.x compatibility.

added 3 commits

• 06839326 - Fix: Coding standard errors flagged up by ESLint.
• cea4df03 - Fix: PHP coding standards.
• 44d51d24 - Fix: Stylelint errors.

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added 4 commits

• af2a1b22 - Fix: Further Stylelint errors.
• e7295e58 - Fix: Further ESLint errors.
• 1af4662a - Fix: Further PHP coding standards.
• 19fa0f7c - Fix: PHPStan errors.

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added 3 commits

• b4b83e11 - 1 commit from branch project:8.x-4.x
• 1c2c2b21 - Merge branch '8.x-4.x' into feature/3408089-gitlab-ci
• baed6a3c - Additional fixes

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added 1 commit

• 78c5feb9 - Phpcs fixes

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added 1 commit

• 47598b4c - additional fixes

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added 1 commit

• 2f09dc4c - phpstan-baseline.neon update

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added 1 commit

• aac25b72 - Extended PHPStan fixes

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added 1 commit

• 389e502f - Additional / Incremental PHPStan fixes

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added 1 commit

• 6daa9093 - phpstan-baseline.neon: add ignored errors

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started a merge train

merged