[BUGFIX] Fix workspace stage-change mail recipient selection

Stage-change notification mails have only been delivered to the last
recipient off the list of possible recipients, because
Utility.convertFormToObject failed to detect checkbox elements
from (other) frames.
In this case the to-be-converted form elements are created on the top
frame (modal contents are placed outside list frame on the top frame).

Due to JavaScript being prototype based, instanceof checks do
only operate per-frame. Prototypes are bound to the current window.
`foo instanceof HTMLInputElement` is basically a shortcut
and equivalent to `foo instanceof window.HTMLInputElement`
while `window != top.window`.

Instead of `instanceof` checks, we switch to a `tagName` comparision
which is usable for cross-frame HTMLElement type detection.

Resolves: #99784
Releases: main, 12.4, 11.5
Change-Id: If9bfd7be1e46d8c04619be3563d3bdba9d9de549
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79953


Tested-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>