[PHP] Use a valid php type for maps

[neclimdul]

This pull request aligns the map types to native array types.

It does this in two steps to show the impact of the change.

1. It maps map to the native array type.
2. It adjusts the syntax to match that expected by psalm, phpdoc, and IDE's that parse phpdoc.

@jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @ybelenko @renepardon

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[neclimdul]

travis failure looks to be trouble talking to npm. Skim of the results seem to show all php tests passing.

[neclimdul]

Most of the changes are documentation. There could be no code instances since map was not a valid keyword/type.

This is kinda the glue for what should be the only technical change. When deserializing this handles but the new generated value and the old map value in case it was being references in some code.

[neclimdul]

I saw some comments that started dealing with this problem by injecting psalm/phpstan comments into the models. This shouldn't be needed anymore and can be removed after this change since it addresses the problem everywhere maps show up.

[wing328]

Travis CI failure not related to this change.

[wing328]

LGTM but I've not tested it locally

[wing328]

@neclimdul thanks for the PR, which has been included in the v5.1.0 release: https://twitter.com/oas_generator/status/1373636654024380423