Deprecate the ArrayAccess interface methods.
Deprecate the ArrayAccess interface methods.

These methods were compatibility shims from the 1.x days. Deprecating
these methods now will make implementing PSR7 simpler as these methods
cannot work on immutable objects.

Refs #9325

mvdriel/cakephp,berrygoudswaard/cakephp,ADmad/cakephp,mvdriel/cakephp,cakephp/cakephp,ndm2/cakephp,garethellis36/cakephp,tenkoma/cakephp,ADmad/cakephp,davidyell/cakephp,ndm2/cakephp,dakota/cakephp,berrygoudswaard/cakephp,garethellis36/cakephp,CakeDC/cakephp,berrygoudswaard/cakephp,tenkoma/cakephp,garethellis36/cakephp,petteyg/cakephp,davidyell/cakephp,cakephp/cakephp,davidyell/cakephp,tenkoma/cakephp,CakeDC/cakephp,mvdriel/cakephp,ADmad/cakephp,ndm2/cakephp,tenkoma/cakephp,mvdriel/cakephp,petteyg/cakephp,cakephp/cakephp,petteyg/cakephp,CakeDC/cakephp,ADmad/cakephp,cakephp/cakephp,ndm2/cakephp,garethellis36/cakephp,dakota/cakephp,dakota/cakephp,petteyg/cakephp,dakota/cakephp,CakeDC/cakephp