[TASK] Render toolbar items in Fluid

Items were previously rendered in BackendController into a large HTML
string that was passed to Fluid. This commit resolves an open TODO and
moves the rendering of the toolbar items to the Topbar partial.

The data attribute `toolbar-identifier` is unused and therefore
removed.

Resolves: #98214
Releases: main
Change-Id: I8fd0e65b9f30aaf27271be47d1b77ea7960d17ba
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75555


Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>