Issue #3420990 by godotislate, smustgrave, alexpott, kim.pepper, longwave: Convert Constraint plugin discovery to attributes

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

Issue #2195083 by Les Lim, ParisLiakos, Xano, gaurav_varshney, ravi.khetri, fago, jain_deepak: Add a dedicated @Constraint annotation class

Issue #2105797 by dawehner, fago, larowlan, effulgentsia, jibran, amateescu, yched: Add CompositeConstraintBase so that constraints involving multiple fields, such as CommentNameConstraint, can be discovered

Issue #2403485 by larowlan, kim.pepper, idebr, fago: Complete conversion of comment form validation to entity validation

Issue #2002158 by klausi, andypost, effulgentsia, vlad.dancer: Convert form validation of comments to entity validation.