Make test pipeline succeed on D11.

Update dev requirements to D11, comment out some incompatible or unused ones + corresponding tests, and add some extra README comments (also in custom_elements_thunder).

Closes #3479689

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• 362e1110 - phpcs + eslint

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• 60ee3487 - more eslint

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• 5097906b - Enable opt-in jobs for extra Drupal/PHP versions.

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merged