[DeadCode] Add RemoveUselessUnionReturnDocblockRector

[TomasVotruba]

Adds a dead-code rule that removes a @return union docblock when a more specific, non-array native return type already covers it.

The docblock adds nothing once the native type is narrower-or-equal, so it is dead.

Before

/**
 * @return bool|mixed|string
 */
protected function setContactToSync($checkEmailsInSF, $lead): false|string

After

protected function setContactToSync($checkEmailsInSF, $lead): false|string

Also handles a union docblock over a single native type, e.g. @return bool|float|mixed|string over : string.

Trigger

• @return type is a union node
• native return type is declared and not array/iterable (kept, may carry element types)
• the docblock type is a supertype of the native type (broader-or-equal)
• empty description (descriptions are kept)

Requires PHP 8.0 (MinPhpVersionInterface -> UNION_TYPES). Registered in the dead-code set via DeadCodeLevel.