Implement new hook_requirements_alter()

Problem/Motivation

When "Trusted reverse proxy" module is enabled and configured correctly, the following error is shown in the "Status report" page:

Notice: Undefined index: severity in trusted_reverse_proxy_preprocess_status_report_page() (line 18 of modules/contrib/trusted_reverse_proxy/trusted_reverse_proxy.module).
trusted_reverse_proxy_preprocess_status_report_page(Array, 'status_report_page', Array) (Line: 287)
Drupal\Core\Theme\ThemeManager->render('status_report_page', Array) (Line: 422)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 201)
Drupal\Core\Render\Renderer->render(Array, ) (Line: 241)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 564)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 242)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object, Object) (Line: 132)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object) (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object)
call_user_func(Array, Object, 'kernel.view', Object) (Line: 142)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object, 'kernel.view') (Line: 163)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 50)
Drupal\ban\BanMiddleware->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 54)
Drupal\trusted_reverse_proxy\StackMiddleware\TrustedReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 708)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

• Download and install a fresh Drupal 9 core
• Download and install the Trusted Reverse Proxy module
• Set the correct configurations in settings.php
• Clear the cache, and visit the status page

Proposed resolution

Adding the "severity" key in the system.install file should be the correct solution, but at least we can start checking if the severity key exists before extracting it on line 17 of the .module file

Remaining tasks

Implement the patch and create the fork

User interface changes

No

API changes

No

Data model changes

No