Mark _c() as Deprecated as people should use _x

[westi]

_c was the old style of including context using |.

_x is better as it uses the native gettext contexts making the translators work easier.

(In [12257]) Mark _c() as Deprecated people should use the new _x() instead. Fixes #11225.

akismet needs an little change from _c() to _x(), too.. (?)

Akismet supports WP versions back to the beginning of time. We should probably branch it at some point and bring it up-to-date with best practices.