Fix setting outBound_option readonly in UI

[mattwire]

Overview

Follow on from #16451

Fix setting the SMTP outbound option readonly.

Before

outBound_option readonly ignored in UI. It was actually working, just not shown as frozen in UI.

After

Elements frozen if you set in civicrm.settings.php:
$civicrm_setting['mailing']['mailing_backend']['outBound_option'] = 5;

Technical Details

readonly attribute was set after adding the element!

Comments

@yashodha you were interested in this? I think it actually already works with the SMTP authentication setting because of #16451

[civibot]

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[colemanw]

Change looks sensible & safe.