[BUGFIX] Load `backend.form` imports in Site Configuration form

In v11, extension authors were able to enhance the site configuration
"TCA" with richtext fields. Since v12, this is not possible anymore due
to the missing `backend.form` imports, required to load necessary
JavaScript modules tagged as such. This commit adds the missing
instruction, allowing to make use of ckeditor and related modules
again.

Resolves: #103115
Releases: main, 12.4
Change-Id: I2fb6d6b917ee99d9616593f118daa72d3f3a460b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83017


Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>