Issue #3239762 by alexpott, daffie: \Drupal\Core\Template\AttributeString::__toString() can cause deprecations in PHP 8.1

Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2505701 by pwolanin, YesCT, akalata: Document SafeMarkup::set and Use htmlspecialchars() directly in Attribute() so we don't bloat the list of safe strings

Issue #2457887 by prateekMehta, stefan.r, rpayanm, alexpott: Use Utility\SafeMarkup class instead of Utility\String for placeholder(), checkPlain(),format() functions

Issue #2227869 by joelpittet: Remove magic printing from Attribute class to make Twig 'without' filter work for attributes.

Issue #2212309 by joelpittet: Drupal\Core\Template and twig.engine docs, coding standards, and unused code cleanup.

Issue #2100329 by cosmicdreams: Use \Drupal instead of Drupal to make IDEs and static code analyse tools happy.

Issue #2003568 by dawehner, jhedstrom: Convert tags,attributes, diff and url validation unit tests to phpunit.

- Patch #1290694 by hefox, tlattimore, c4rl, chx, tim.plunkett, tstoeckler, msonnabaum: provide consistency for attributes and classes arrays provided by template_preprocess().