SimpleCaptcha::storeCaptcha is passing an array to CaptchaStore::store,
so allow to store arrays
The implemenation allows to store mixed
Change-Id: I24bb8d852475c054c179a8c8d48417d8983797fa
And also, avoid deprecated use of wfMemcKey() global function. So,
create a BagOStuff cache object and use. ::getMainStashInstance() was
deprecated in 1.28 and the MainObjectStash service was marked available.
Currently, extension requires MediaWiki 1.31 at the minimum so the
replacement is safe.
Change-Id: I01494a2dcbcbf30fe9960fdfb55b4912a648c7b6
