Use \OCP\DB\Types instead of column type strings in migrations

[ChristophWurst]

Is your feature request related to a problem? Please describe.

Our migrations are filled with literal strings for column types while there are class constants for them.

Describe the solution you'd like

Replace literals with constants, e.g. \OCP\DB\Types::STRING instead of 'string'

Describe alternatives you've considered

No response

Additional context

No response