Update default instance description

EDIT: See updates below.

On one hand I'm in no position to be making marketing decisions. On the other hand, this tagline hasn't been updated in a while, and maybe it's good to have a fresh set of eyes on it.

Old:

A Pleroma instance, an alternative fediverse server

New:

Pleroma: An efficient and flexible social media server

It's notable that the word Fediverse is intentionally no longer in the tagline. More importantly, Pleroma is not an alternate solution, but the preferred one, so we make the tagline reflect that.