[Backport] Issue Fixed: #8086: Multiline admin field is broken

[eduard13]

Original Pull Request

#20371

Description (*)

Fixed Issues (if relevant)

1. Multiline admin field is broken #8086: Multiline admin field is broken
2. Multiline field is broken #18115

Manual testing scenarios (*)

1. Add field type="multiline" in system config in admin.
2. Save values in multiple fields

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @eduard13. Thank you for your contribution
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[magento-engcom-team]

Hi @rogyar, thank you for the review.
ENGCOM-4379 has been created to process this Pull Request

[ihor-sviziev]

Hi,

Closing in favor to #21561 (create by author of original PR)

[ghost]

Hi @eduard13, thank you for your contribution!
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