Announce that -spanner will be moving in N days

[devjgm]

For Spanner, I think N = 60

We want to give customers a heads up that the -spanner repo will be moving into the monorepo at -cpp. We should cut a release and and include an announcement that this will be happening. We may also want to drop a note in our slack channel and anywhere else we can, like on the README.md.

[devjgm]

Tasks:

• Update https://github.com/googleapis/google-cloud-cpp-spanner/blob/master/README.md to say that -spanner is moving into the -cpp monorepo and link to Move (back) to a Monorepo #3612
• Include a similar blurb in Spanner's April Release Notes. Tagging @scotthart who will be handling the next Spanner release

[devjgm]

Assigning this to @scotthart since he'll do the next announcement.