docs: Update upgrade guide to 4.0

[Zfinix]

Summary

Updates the upgrade guide from 3.5 to 4.0 (title, headings, prose, version pins, and the migration tag). The 3.4 from-version and the Dart SDK constraints are left unchanged.

The filename and URL slug keep upgrade-to-three-five so the existing inbound links from the legacy-auth guide don't break. Happy to rename the file in a follow-up if we want the URL to match.

Part of #584

[Swiftaxe]

Looks good!

One thing left: The guide is missing how to set up the MCP servers. This is done automatically when creating new serverpod projects, but when upgrading it is currently manual, unless we offer a command to run in the CLI.

It says: "Some editors, like Cursor, require enabling the MCP server in their settings.", but the user gets no help in how to set this up for their IDE, and has to research this themselves.

[Swiftaxe]

A new command was merged yesterday, serverpod create . which improves the upgrading experience.

For instance, upgrading a project without migrations will show the Database and IDE options in the TUI.
Upgrading a project that already contains migrations will only show the IDE options in the TUI.

The TUI will then give options to select IDE which then install skills, and configure MCP.

serverpod/serverpod#5341

[Zfinix]

Looks good!

One thing left: The guide is missing how to set up the MCP servers. This is done automatically when creating new serverpod projects, but when upgrading it is currently manual, unless we offer a command to run in the CLI.

It says: "Some editors, like Cursor, require enabling the MCP server in their settings.", but the user gets no help in how to set this up for their IDE, and has to research this themselves.

Can we add this to a new PR