Issue #3446528: Run tests on 11.x and declare compatibility with it

.gitlab-ci.yml

composer.json

modules/project_browser_devel/project_browser_devel.info.yml

...er_source_example/project_browser_source_example.info.yml

project_browser.info.yml

project_browser.services.yml

src/ComposerInstaller/Validator/CoreNotUpdatedValidator.php

...poserInstaller/Validator/PackageNotInstalledValidator.php

src/Controller/InstallerController.php

src/EventSubscriber/UpdateFixtureSubscriber.php

tests/fixtures/drupalorg_jsonapi/3.json

tests/fixtures/drupalorg_jsonapi/4.json

tests/fixtures/drupalorg_jsonapi/5.json

tests/fixtures/drupalorg_jsonapi/6.json

tests/fixtures/drupalorg_jsonapi/list-1.json

tests/fixtures/drupalorg_jsonapi/list-10.json

tests/fixtures/drupalorg_jsonapi/list-11.json

tests/fixtures/drupalorg_jsonapi/list-12.json

tests/fixtures/drupalorg_jsonapi/list-13.json

tests/fixtures/drupalorg_jsonapi/list-14.json