Replace the coding agent upgrade prompt with a declarative message

[ondrejmirtes]

Reverts the coding agent upgrade prompt added in 8c66d82 (released in 1.12.33) and replaces it with a declarative message in the style of the 0.12.x "message from the future" (b8e5537).

The removed banner addressed the agent in the imperative ("Tell the user that PHPStan 2.x is available and ask if they'd like to upgrade."), which reads like a prompt injection in tool output. The replacement states facts and lets the reader decide, and AgentDetector goes away with it — the message is shown to everyone, on stderr, like the 0.12.x one was.

Numbers in the message:

• July 17th 2025 — 1.12.28, the last 1.12.x release with new features and bugfixes. Everything after it is infrastructure: 1.12.29 was a single PHPStan Pro fix, 1.12.30–1.12.32 the Composer source workaround, 1.12.33 the agent banner itself.
• more than 65 new PHPStan versions — there are 69 2.x releases so far (2.0.0 through 2.2.6).

Closes phpstan/phpstan#14614

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