Add phpdoc return typehints for JsonDocumentType class

[hhamon]

The main goal is to suppress the following warning messages raised by Symfony:

2022-01-18T07:23:10+00:00 [info] User Deprecated: Method "Doctrine\DBAL\Types\Type::convertToDatabaseValue()" might add "mixed" as a native return type declaration in the future. Do the same in child class "Dunglas\DoctrineJsonOdm\Type\JsonDocumentType" now to avoid errors or add an explicit @return annotation to suppress this message.

2022-01-18T07:23:10+00:00 [info] User Deprecated: Method "Doctrine\DBAL\Types\Type::convertToPHPValue()" might add "mixed" as a native return type declaration in the future. Do the same in child class "Dunglas\DoctrineJsonOdm\Type\JsonDocumentType" now to avoid errors or add an explicit @return annotation to suppress this message.

2022-01-18T07:23:10+00:00 [info] User Deprecated: Method "Doctrine\DBAL\Types\Type::requiresSQLCommentHint()" might add "bool" as a native return type declaration in the future. Do the same in child class "Dunglas\DoctrineJsonOdm\Type\JsonDocumentType" now to avoid errors or add an explicit @return annotation to suppress this message.

2022-01-18T07:23:10+00:00 [info] User Deprecated: Method "Doctrine\DBAL\Types\Type::getName()" might add "string" as a native return type declaration in the future. Do the same in child class "Dunglas\DoctrineJsonOdm\Type\JsonDocumentType" now to avoid errors or add an explicit @return annotation to suppress this message.

[dunglas]

This is already handled by #102

[hhamon]

@dunglas yes and no because #102 enforces PHP 8.0 in composer.json file.

I think we need a last 1.x version that fixes deprecations and create a 2.x branch to support PHP 8.0+

[ddeboer]

Makes sense to merge this first and tag 1.x, then merge #102 and tag that as 2.0. (I’ll rebase #102 after this is merged.)

[dunglas]

We don't have to release a new major version to bump PHP. I'll merge this one, tag, then bump the version into a new minor release.

[dunglas]

Thanks Hugo!