chore(release) Prepare version v5.1

Author: njoubert-cleverage

CHANGELOG.md

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 Latest
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+v5.1
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 ## Changes
 * [#190](https://github.com/cleverage/process-bundle/issues/190) Update quality stack: use Rector `withComposerBased()` sets (removed `SYMFONY_64` / `PHPUNIT_100` sets), declare used Symfony packages and PHPUnit range in composer.json, apply quality tools fixes
 * [#145](https://github.com/cleverage/process-bundle/issues/145) Add missing documentations: reference pages for every Task & Transformer, ConditionTrait & GenericTransformer, complete guides and cookbooks. Harmonize and fix existing documentation.