Backporting: https://github.com/doctrine/dbal/pull/2816

Backporting https://github.com/doctrine/dbal/pull/2905

Backporting https://github.com/doctrine/dbal/pull/2907

Serial fields already have a default value. By opting out of `notnull`
serial fields will just set the next value in the sequence.

Postgres has a `SERIAL` data type to allow shorthand for
`nextval('sequence_name')`, but DDL like this:

    CREATE TABLE example (id SERIAL)

Cannot be generated in the DBAL schema APIs.

Backporting https://github.com/doctrine/dbal/pull/2861

Backporting https://github.com/doctrine/dbal/pull/2855

I know, this is REALLY nasty but that was the only way I've found to
make it work properly.

And then we don't need to wait till v3 to not require comments on the
JsonType.

On 31d01283 we made JsonType the default
one, which caused some unexpected results with the comparator.

This tests validates that if users have a JSON column but are using the
JsonArrayType we'll just add a comment to the column.

Reference: https://github.com/doctrine/dbal/pull/2782

Backport #2821

Added upgrade notice about BC break.
Cleanups

Changed DBALException::invalidPlatformSpecified() exception to be more detailed about what was given.

Fixed cs issues

Backport #2785

Backport #2794 to 2.6.1
ge aborts

#2794 allowing `DateTimeImmutable` instances when converting in mutable date types - reverts unintentional BC break

#2794 adding tests verifying the regression, `DateTimeImmutable` should STILL work, even if unintentionally