Stop passing E_USER_ERROR to trigger_error() on PHP 8.4

Problem/Motivation

RFC accepted https://wiki.php.net/rfc/deprecations_php_8_4#deprecate_passing_e_user_e...

Steps to reproduce

core$ git grep E_USER_ERROR
core/includes/errors.inc:34:    E_USER_ERROR => ['User error', RfcLogLevel::ERROR],
core/includes/errors.inc:70:    $to_string = $error_level == E_USER_ERROR && str_ends_with($caller['function'], '__toString()');
core/lib/Drupal/Component/Diff/DiffFormatter.php:137:        trigger_error('Unknown edit type', E_USER_ERROR);
core/lib/Drupal/Component/Utility/ToStringTrait.php:20:      trigger_error(get_class($e) . ' thrown while calling __toString on a ' . static::class . ' object in ' . $e->getFile() . ' on line ' . $e->getLine() . ': ' . $e->getMessage(), E_USER_ERROR);
core/lib/Drupal/Component/Utility/ToStringTrait.php:22:      // E_USER_ERROR, we terminate execution. However, for test purposes allow
core/modules/big_pipe/src/Render/BigPipe.php:375:          trigger_error($e, E_USER_ERROR);
core/modules/big_pipe/src/Render/BigPipe.php:411:          trigger_error($e, E_USER_ERROR);
core/modules/big_pipe/src/Render/BigPipe.php:562:            trigger_error($e, E_USER_ERROR);
core/tests/Drupal/Tests/Core/StringTranslation/TranslatableMarkupTest.php:82:    $this->assertEquals(E_USER_ERROR, $this->lastErrorNumber);

Proposed resolution

clean-up usage or make it conditional

Remaining tasks

- file MR and change record
- commit

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet