Add fixers for refactorings due to phpunit 7/8 deprecations

[phil-davis]

See owncloud/core#36501 for description of the phpunit deprecations and associated php-cs-fixer fixes.

Add these to the regular code-style checks so that people who add new unit tests will realise to use the newer phpunit functions/methods...

In composer.json require friendsofphp/php-cs-fixer 2.16 or later, just to make sure that all the fixers will be available.

[phil-davis]

Note: before actually releasing this we need to have merged owncloud/core#36501 so as not to immediately break core code-style CI.
And also release at a time when we are happy that we have either:

• applied these fixers already in all the apps or;
• we are happy to find out in the nightly CI which apps now fail the checks and to fix them

Or we can make this a major version release, in which case core and each app can be bumped to require owncloud/coding-standard ^2.0 as we want/are able to apply the fixers.

[phil-davis]

@micbar @PVince81 @DeepDiver1975 or whoever manages this these days.
Note: I do not have any write access to this repo. I cannot add labels, or call in reviewers...

[micbar]

@phil-davis Try now :-)

[phil-davis]

I have access now - thanks.

[phil-davis]

Or we can make this a major version release

I am now inclined to make this a major version release, so that we can implement it in each app repo in a controlled way whenever we like.

Review please.

[phil-davis]

Closing in favour of #20