D11 compatibility

Perhaps would be nice to clean up the situation around #3309931: Drupal 10 compatibility issues and close it first, then continue with focusing on compatibility with Drupal 11.

Hi @japerry
The CI is green now. All PHPUnit tests are passing. FYI: I've reverted the two commits: 12cf6068 & 94cdba03 as I don't think it's good idea to disable ConfigSchema checking. To fix this, I've added an event_subscriber to update config value at runtime.

For Drupal 11, following two PHPUnit tests were failing:

1) Drupal\Tests\ctools_views\Functional\CToolsViewsBasicViewBlockTest::testHideFields
Failed asserting that 0 matches expected 5.
/builds/issue/ctools-3445547/modules/ctools_views/tests/src/Functional/CToolsViewsBasicViewBlockTest.php:250
2) Drupal\Tests\ctools_views\Functional\CToolsViewsBasicViewBlockTest::testOrderFields
Failed asserting that 0 matches expected 4.
/builds/issue/ctools-3445547/modules/ctools_views/tests/src/Functional/CToolsViewsBasicViewBlockTest.php:314
ERRORS!

When I debugged further, I found that the test was failing due to following schema validation check:

Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for block.block.views_block__ctools_views_test_view_block_fields with the following errors: block.block.views_block__ctools_views_test_view_block_fields:settings.fields.id missing schema, block.block.views_block__ctools_views_test_view_block_fields:settings.fields.name missing schema, block.block.views_block__ctools_views_test_view_block_fields:settings.fields.age missing schema, block.block.views_block__ctools_views_test_view_block_fields:settings.fields.job missing schema, block.block.views_block__ctools_views_test_view_block_fields:settings.fields.created missing schema, block.block.views_block__ctools_views_test_view_block_fields:settings.fields.name_1 missing schema in Drupal\Core\Config\Development\ConfigSchemaChecker->onConfigSave() (line 101 of core/lib/Drupal/Core/Config/Development/ConfigSchemaChecker.php).

I wasn't sure on how to fix it, so I temporary disable the schema validation check (same you did earlier). If you know, how to fix it properly, that would be great.
Thanks.

This issue caused a break in #3454239: PageBlockDisplayVariant::__sleep() is no longer compatible with parent signature from ctools leading to PHP fatals. Mind reviewing that @japerry, since you're also a maintainer there.