Fix: persists "Supported Gateways" field on the settings of the "Subscription Payment Failed" email

[glaubersilva]

Resolves GIVE-789

Description

This PR fixes a bug that prevents the "Supported Gateways" field be saved on the settings of the "Subscription Payment Failed" email. The problem was introduced in the following PR: #6237

So, the solution was to change how to set the "selected" attribute of the select options to make it work in the same way as before the changes introduced in the PR mentioned above.

Affects

The multiselect fields spread on the codebase.

Visuals

https://www.loom.com/share/6eabb134dc0940ec8af9d893d079ac9c?sid=e68abf36-b943-40ed-b512-9ade95bbfa95

Testing Instructions

1. Go to Give > Settings > Emails > Subscription Payment Failed page;
2. On the "Supported Gateways" field select a few gateways;
3. Save the changes and make sure the selected gateways are saved.

Additional test: Please, follow the test instructions from this related PR and make sure the changes introduced there keep working as before.

Pre-review Checklist

• Acceptance criteria satisfied and marked in related issue
• Relevant @unreleased tags included in DocBlocks
• Includes unit tests
• Reviewed by the designer (if follows a design)
• Self Review of code and UX completed

[alaca]

👍

[rickalday]

Passed manual QA tests.