docs: State the PostgreSQL 16 minimum and surface the file storage null change

[FXschwartz]

A 4.0.4 user on Postgres 14 hit a failing integrity check and could not create a repair migration, and separately missed that retrieveFile now throws instead of returning null. Both were documentation gaps. See serverpod/serverpod#5843.

• Installation and quickstart prerequisites: a self-managed PostgreSQL must be version 16 or later.
• Upgrade to 4.0: a "Before you start" bullet for the Postgres version, and the production note now says to pick engine version 16 or later.
• Migrate from legacy auth: "Postgres 14 or later" corrected to 16.
• Breaking changes in 4.0: the file storage section is retitled to say the methods throw instead of returning null, the lookup table mentions null checks, and the intro notes that an existing null check still compiles with only an analyzer warning.

[developerjamiu]

Apply the same changes to the five pages in versioned_docs/version-4.0.0/. 4.0.0 is the version served at /, so without them this only shows under /next.

[developerjamiu]

Add the minimum to the #### PostgreSQL section of docs/06-concepts/01-server-fundamentals/03-configuration.md too, in both copies. That's where readers are told to connect an external PostgreSQL in production, and it gives no version. For example: "The external PostgreSQL must be version 16 or later."