Carries the upgrading page's title and pin list to 0.23

[johnnyt]

Summary

The upgrading page gained its "0.22 to 0.23" section, but its title, opening paragraph, pin chain and schema sentence still stopped at 0.22. This carries those four sites to 0.23 and changes nothing else on the page:

• the title reads "Upgrading a host from 0.13 to 0.23";
• the opening paragraph moves a host from 0.13.0 to 0.23.0;
• the pin chain ends with "~> 0.23.0" (the README's install pin is already ~> 0.23.0);
• the schema sentence says no release from 0.18.0 through 0.23.0 adds a migration, and neither does the 0.23.1 patch.

Refs: sp-4cy9

Gate

This branch touches only docs/upgrading.md, outside the gate's build paths, so there is no gate to run (the repo's commit authority row: a change touching no Elixir code commits on review of the diff alone). CI runs the full gate on this request. No changelog fragment: documentation only, per changelog.d/README.md.

Review

Own review of the diff against the bead: one file changed, the four sites named above and no others; the "0.22 to 0.23" section and its "0.23.1" subsection are untouched. The schema claim was checked against the code: StatifierPersistence.Ecto.Migrations computes @current_version from its migrations map, whose modules are V01 through V08 under lib/statifier_persistence/ecto/migrations/, and no commit since the v0.22.0 tag touches that directory or migrations.ex. The "every later release keeps it there" sentence on the statifier floor still holds: mix.exs pins {:statifier, "~> 2.9"}.