Allow namespaced migration files

[dereuromark]

I wonder if we could use cakephp/phinx#1820 to namespace our migration classes
They are the only code I still have to exclude from sniffers so far:

FOUND 2 ERRORS AFFECTING 1 LINE
----------------------------------------------------------------------------------------------------------------------------------------
 6 | ERROR | Class name "MigrationLanguages" doesn't match filename, expected "20201014000218_MigrationLanguages"
 6 | ERROR | Each class must be in a namespace of at least one level (a top-level vendor name)

With the namespace way it would also probably be a bit cleaner and less conflicting regarding the names.

Right now I always prefix them with Migration to avoid naming clashes/collisions with existing top level classes.

[dereuromark]

The CS issue is "hacked" for now with cakephp/cakephp-codesniffer#320

We should still looking into a phinx solution that allows namespace migration class names, though.
According to https://github.com/cakephp/phinx/blob/master/docs/en/namespaces.rst#psr-4-compliance this should already be possible, maybe sth for Cake 4.2+?