JsonLogTest deprecation with PHPUnit version 10

This looks good to me.

Potentially setting core_version_requirement to ^10.3 || ^11 might help for the next stable release as well in case anyone one who hasn't gotten to 10.3 tries to update and runs tests. That could also be done in the other issue as well.

@scott_earnest Great work! But this will not be (backwards)compatible with PHPunit 9 I presume?
In that case we'll probably have to create an new D11 line (dev branch + major version). Will do for sure, but slightly later.

But this will not be (backwards)compatible with PHPunit 9 I presume?

It should be easy to figure out by adding test previous major into gitlab CI yaml. I needed to add that anyway to most of my modules because of the flip to testing Drupal 11 as the current major.

Also the test run from 3 months ago passes on PHPUnit 9:

PHPUnit 9.6.20 by Sebastian Bergmann and contributors.
..................................                                34 / 34 (100%)
Time: 00:02.545, Memory: 6.00 MB