Use hook_requirements_alter() to only silence the warning, not take over the whole update.module namespace

Problem/Motivation

See discussion at #2869592-41: Disabled update module shouldn't produce a status report warning → for a description of some of the problems of the current way this module is implemented. However, since #309040: Add hook_requirements_alter() → was committed to core (available in 9.5.0 and later), there's now a clean and simple way to achieve the desired effect here, use hook_requirements_alter().

Steps to reproduce

Proposed resolution

Create a 2.0.x branch with the whole new (much simplified) way to silence the warning about not having core's update.module installed using hook_requirements_alter()

Remaining tasks

User interface changes

API changes

Data model changes