Update param type for PHP Internal Mapfield::offsetGet

[M1ke]

This is a minor doctype change for the PHP codebase, Internal\Mapfield::offsetGet method

The param was typed as object which is invalid as when items are set in the container the key passes through checkKey where it enforces the key must be various classes of integer, bool or string. Thus I have replaced it with a union type (in future this can be a proper PHP level typehint, but for now only doctype)

I encountered this whilst using a client that consumes protocol buffers, and found that my IDE was telling me that $item->offsetGet('key') was invalid, yet on running the code I got the expected output from the Map container.

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[M1ke]

@googlebot I signed it!

[M1ke]

Thanks for the approval, the failing merge check doesn't look like something related to this work but not sure how to resolve that?