Block plugins subclassing BlockBase::blockAccess() must return AccessResult instead of a bool

In HEAD, this convenience method still returns a boolean. But that means the necessary access cacheability metadata is missing. By requiring an AccessResultInterface object to be returned, that problem is solved.

BEFORE

  protected function blockAccess(AccountInterface $account) {
    return $account->hasPermission('a permission');
  }

AFTER

  protected function blockAccess(AccountInterface $account) {
    return AccessResult::allowedIfHasPermission($account, 'a permission');
  }