Tutorial: drop the Next.js 16 downgrade workaround

Fedify CLI 2.1.8 ships with `@fedify/next` 2.1.8, whose peer
dependency on `next` was widened to `>=15.4.6 <17`, so
`fedify init -w next` against the current `create-next-app` now
installs Next.js 16 cleanly without the manual `package.json` edit
the tutorial used to describe.

Remove the WARNING callout and the pinning block, and update the
prereq sentence in [*Installing the `fedify` command*] to require
CLI 2.1.8 or higher (the version that gained Next.js 16 support)
rather than the 2.2.0 it previously named.  Also bump the Next.js
version printed in the sample *npm run dev* console output from
15.5.15 to 16.2.4 so it matches what a reader actually sees.  The
`fedify-dev/threadiverse` example repository has been updated to
the same dependency versions in a paired commit.

[*Installing the `fedify` command*]: https://fedify.dev/tutorial/threadiverse#installing-the-fedify-command

Resolves #713.

Author: dahlia

docs/tutorial/threadiverse.md

@@ -146,10 +146,9 @@ Check that it works:
 fedify --version
 ~~~~
 
-Make sure the version number is 2.2.0 or higher.  Older versions of the CLI
-don't know how to scaffold a Next.js project, and the `@fedify/fedify`
-release we depend on for outbound interop with Lemmy was cut alongside
-CLI 2.2.
+Make sure the version number is 2.1.8 or higher.  Older versions of the
+CLI don't know how to scaffold a Next.js 16 project, and this tutorial
+was written against the shape that CLI 2.1.8+ produces.
 
 ### `fedify init` to initialize the project
 
@@ -173,31 +172,6 @@ serve ActivityPub.  The options mean:
  -  `-m in-process`: run Fedify's background message queue in-process instead
     of on an external broker (Redis, RabbitMQ, and so on).
 
-> [!WARNING]
-> At the time of writing, `create-next-app` (which `fedify init` runs under
-> the hood) defaults to Next.js 16, but `@fedify/next` 2.1.x still requires
-> Next.js 15.4 or later in the 15.x line.  After `fedify init` finishes it
-> will try to run `npm install` and fail with an `ERESOLVE` error.  Open
-> *package.json*, change the `next`, `react`, `react-dom`, and
-> `eslint-config-next` versions so they point at Next.js 15.5.x, then run
-> `npm install` again:
->
-> ~~~~ json
-> "dependencies": {
->   "next": "^15.5.15",
->   "react": "^19.2.4",
->   "react-dom": "^19.2.4",
->   ...
-> },
-> "devDependencies": {
->   ...
->   "eslint-config-next": "^15.5.15",
->   ...
-> }
-> ~~~~
->
-> This workaround will go away once Fedify ships support for Next.js 16.
-
 After a moment your working directory will contain something like this:
 
  -  *app/* — Next.js App Router pages and layouts
@@ -239,7 +213,7 @@ The dev server will keep running until you press
 <kbd>Ctrl</kbd>+<kbd>C</kbd>:
 
 ~~~~ console
-  ▲ Next.js 15.5.15
+  ▲ Next.js 16.2.4
   - Local:        http://localhost:3000
   - Network:      http://192.168.x.x:3000