[BUGFIX] Use correct variable name for confirmation of FormEngine reload

The confirmation dialog when FormEngine requests an update never came
up in FlexForms as the array key used in
`TYPO3\CMS\Backend\Form\Behavior\ReloadOnFieldChange->toArray()` didn't
match the key in the `typo3-backend-form-reload` handler in FormEngine
JavaScript, which is solved in this commit.

Resolves: #97804
Related: #91787
Releases: main, 11.5
Change-Id: I1b95e836408a579d1b3e075f7614d7907bf94a37
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75085


Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>