Support for single connections to the database

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What does this MR do?

1. Adds back the workaround for single connections to the database.

   • Previously this was separately in the Omnibus, Rails, and Helm chart repos, for simplicity I've reinstated it in the upgrade notes. This entire page will get dropped roughly when 19.0 is released.

2. Adds information about pgbouncer and signposts the client connection limit change on PostgreSQL. Related: Issues with lots of DB connections (#463715)

3. Support for single database connections continues, and now removal is 19.0. Bumped the deprecation entry.

4. 17.0 included encouragement to run decomposed databases, but that doesn't work yet with Geo. Documented that.

Related issues

#463715

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