fix: stop using deprecated IAppContainer::getServer()

[miaulalala]

Summary

• App::getContainer() is now typed as returning Psr\Container\ContainerInterface instead of the deprecated IAppContainer (nextcloud/server@1d55835d39, "refactor: Remove most usages of IAppContainer and IServerContainer"), so psalm flags ->getServer() as undefined on the narrower type in current Renovate PRs bumping nextcloud/ocp.
• getServer() has been deprecated since Nextcloud 20 anyway, and the app container's get() already resolves core/server services directly (same pattern used throughout DIContainer itself), so it was redundant.
• Replaced both call sites in registerActivityConsumer() and registerNotifier() with direct $c->get(...) / $this->getContainer()->get(...) calls.

Test plan

• Ran composer update nextcloud/ocp --with-dependencies to pull the same OCP commit CI uses, then composer run psalm — no errors (previously 3 UndefinedInterfaceMethod/MissingPureAnnotation errors on getServer())
• CI green on this PR
• AI assisted