Fix deprecated CLOB type declaration in SQL Server platform

[deeky666]

According to the SQL Server documentation the text type declaration is deprecated and will be removed in the future. It is encouraged to use varchar(max) instead. This has been applied to SQLServerPlatform::getClobTypeDeclarationSQL() by this PR.
The varchar(max) declaration is BC for all SQL Server versions supported by Doctrine btw.

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[beberlei]

@deeky666 Does SchemaManager detect VARCHAR(MAX) as text field compared to string?

[deeky666]

@beberlei Yes the schema manager is implemented to do so. But I can add an explicit functional test for this if you want :)

[deeky666]

As far as I remember textis internally mapped to varchar(max) by SQL Server anyway thus always retrieving varchar type from database with schema manager. I think I investigated once on how types are returned from DB and encountered this.