[BUGFIX] Make paste content button accessible via keyboard

The paste content button in the page module was not
accessible via the keyboard because of it being an anchor
element without a `href` attribute (so no tabstop).

The button is therefore now changed to use a button
tag, allowing it to be accessed via the keyboard.

Resolves: #92653
Releases: master, 10.4
Change-Id: Ied665d2899b6364046ff7920dfa9dcb006de0d87
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/66238


Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Daniel Haupt <mail@danielhaupt.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Daniel Haupt <mail@danielhaupt.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>