Connectors: Register Akismet Anti-Spam as a connector.

Akismet is bundled with WordPress core, but it wasn't appearing on the Connectors screen even when present on the file system.

Partially reverts [62210], bringing back the main part of [62193].
Follow-up to [62210], [62193].
Developed in https://github.com/WordPress/wordpress-develop/pull/11688.

Props audrasjb, bluefuton, davidbaumwald, desrosj, eclev91, gziolo, johnbillion, jorbin, jorgefilipecosta, lukecarbis, matt, matveb, peterwilsoncc, westonruter.
See #65012.

git-svn-id: https://develop.svn.wordpress.org/trunk@62310 602fd350-edb4-49c9-b593-d223f7449a82