Revert "Issue #2560641 by alexpott, lauriii, Xano, borisson_, ianthomas_uk: Remove all usages SafeMarkup::checkPlain() from render arrays"

This reverts commit 31007609.

Issue #2560641 by alexpott, lauriii, Xano, borisson_, ianthomas_uk: Remove all usages SafeMarkup::checkPlain() from render arrays

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

Issue #2349773 by mirom, Zekvyrin, subhojit777, lauriii: Twig Double escaping on modules' available updates page

Issue #2343661 by larowlan, tim.plunkett | xjm: Rename l() to _l() and url() to _url(), and document replacements.

Issue #2308821 by tim.plunkett: Replace FormErrorInterface with $form_state->setErrorByName() and $form_state->setError().

Issue #2010246 by tim.plunkett, tkuldeep17, plopesc, InternetDevels, pfrenssen, googletorp: Convert update_manager_install_form, update_manager_update_form, update_manager_update_ready_form to the new form interface.