Support Symfony4 and core 10.4

[phil-davis]

core recently moved from Symfony 3 to 4 - core release 10.4.0 should have that.

composer.json here in data_exporter mentions various Symfony components at
version 3.4

data_exporter needs to support ownCloud v9.*

Somehow the CI is running OK here against current core and against latest 10.3.2 release. That is a bit surprising, since those two have different Symfony major versions.

Need to investigate if all is well leaving Symfony at v3.4 here in data_exporter, or if we will have to manage releases to support core 9.* through 10.3.2, and different to support core 10.4.* onwards.

[phil-davis]

@micbar FYI

We should do some investigation. But maybe all is well.

[micbar]

@phil-davis Complicated topic.

We need support for oc 9 only on the exporter side.

[phil-davis]

I suspect that this works OK like it is now, because the acceptance tests pass.
But it just needs someone to have "a look around" and make sure.