Clean up Drupal 7 leftovers, coding standards, and typos

Problem/Motivation

Some code is still left over from Drupal 7, and the phpcs and cspell jobs fail in the pipeline.

• The help page is empty. customerror_help() still has the Drupal 7 signature and looks for admin/help#customerror.
• The settings form injects path_alias.manager and never uses it. It's the only reason the module depends on path_alias.
• The 403 and 404 sections of the settings form are fieldsets with #collapsible and #collapsed, which don't do anything anymore.
• The settings form calls t().
• phpcs flags an unused Node import and an undocumented $request parameter in the controller.
• LoadTest uses the @group annotation, which PHPUnit 12 no longer supports.
• cspell flags two typos: "configiuration" in the README and "unfamilar" in the redirect list help text.
• cspell also flags the names of the maintainers in the README.

Proposed resolution

• Update customerror_help() to the current signature so the help page shows the module description.
• Remove the alias manager and the path_alias dependency.
• Change the fieldsets to details elements.
• Use $this->t().
• Remove the import and document the parameter.
• Use #[Group('customerror')] on LoadTest.
• Fix the two typos.
• Add the names to a .cspell-project-words.txt.

Tests:

• The help page shows the module description.
• The body renders as HTML with scripts stripped.