[TASK] Switch to json_encode for tx_cms_showpic parameters

Scalar values sent via HTTP query parameters to ShowImageController
are using `json_encode` instead of `unserialize`. The parameter
stream is still secured with an HMAC before being deserialized.

Resolves: #91509
Releases: master, 10.4, 9.5
Change-Id: I81b8d5a10a79536592b105c838470238e14b7dca
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64589


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>