Issue #3421016 by quietone, godotislate: Convert MigrateProcessPlugin plugin discovery to attributes

Issue #3222251 by bbrala, longwave: [November 8, 2021] Replace all isset constructs with the null coalescing operator

Issue #2989893 by ankithashetty, srilakshmier, quietone, firfin, longwave: Remove redundant source: key in Substr example

Issue #3164652 by jungle, quietone, longwave: Update modules/migrate/src/Plugin/migrate/process/Substr.php to remove "skłodowska" from misc/cspell/dictionary.txt

Issue #2849669 by alexpott, pwolanin, Munavijayalakshmi, dawehner: Fix \Drupal\Component\Utility\Unicode() because of the Symfony mbstring polyfill

Issue #2845490 by Jo Fitzgerald, wturrell, quietone, phenaproxima, xjm: Add documentation to Substr process plugin

Issue #2682461 by arunkumark, jonhattan, quietone, benjy, xjm, RavindraSingh, catch: Add links to documentation in all migrate process plugins