Properly deprecate AssertLegacyTrait::assert

Let's start with a deprecation trigger and see what neeeds to be fixed. This is a good novice task - note though that the replacement should use the best assertion possible, not just ::assertTrue. For example, this

return $this->assert(empty($links), $message, $group);

should be converted as

return $this->assertEmpty($links, $message, $group);

while

$this->assert(strpos($content, implode(' ', $expected)) !== FALSE, 'Form alter hooks executed in the expected order.');

as

$this->assertStringContainsString(implode(' ', $expected), $content, 'Form alter hooks executed in the expected order.');

Closed duplicate is more appropriate in this case.