Renamed existing automation poll to prepare for new one (#28132)

towards https://linear.app/ghost/issue/NY-1286

**What?** Rename "poll" to "welcome email automation poll". No expected
user impact.

**Why?** We plan to add a new polling system (see [NY-1286]) but keep
the current one for awhile. Let's rename the current one so we can have
both, and make the distinction clear.

[NY-1286]: https://linear.app/ghost/issue/NY-1286