[BUGFIX] Ensure type inheritance compatibility for RouteResultInterface

PageArguments and SiteRouteResult implementing the
\ArrayAccess interface indirectly through implementing
RouteResultInterface, but do not have set parameter
and return type for some methods. This could not be done
as mixed is only availabe since PHP 8.0.

PHP 8.1 implemented concrete return types for internal
classes, interfaces and methods which triggers deprecation
messages, unless they are part on an inheritance.

This patch adds the #[\ReturnTypeWillChange] attribute
to suppress the message for PHP 8.1, but ensure the
compatibility for PHP 7.4.

Further @todo's are added to remove #[\ReturnTypeWillChange]
and add concrete return types and parameter types as
breaking change in v12.

Resolves: #95746
Releases: master
Change-Id: I584e56295d43d063b9cce05377f5ab3c1641b00a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71949


Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>