由于软件包保留，无法将 18.04 升级到 20.04/20.10

我正在尝试从 18.04 升级到 20.10。我的理解是，我首先需要更新到 20.04，然后再更新到 20.10。我正在按照此页面上的说明进行操作。

当我运行时，sudo apt upgrade我收到此消息：

The following packages have been kept back:   
  frei0r-plugins libssh-4 
0 upgraded, 0 newly installed, 0 to remove and 2 not upgraded.

我不知道为什么这些被阻止。我隐约记得安装了第一个包，但不知道我安装了什么需要第二个包。

当我跑步时，sudo apt dist-upgrade我得到了相同的确切信息。

当我继续时，sudo apt do-release-upgrade我收到此错误：

Checking for a new Ubuntu release
Please install all available updates for your release before upgrading.

好的，我想这与保留的包裹有关？我读了一个地方，我可以将它们取消标记为已保留，但这不起作用：

$ sudo apt-mark unhold frei0r-plugins libssh-4
frei0r-plugins was already not hold.
libssh-4 was already not hold.

在另一个地方，我读到sudo aptitude safe-upgrade可能会解决问题。它安装了一些 i386 软件包，但之后我在尝试升级时仍然收到与以前相同的消息/错误。

我还读到我应该运行apt list --upgradable并且sudo apt-get install每个包都单独列出。但是当我尝试我得到这个错误时：

Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 frei0r-plugins : Depends: libopencv-objdetect4.2 but it is not going to be installed
                  Depends: libopencv-video4.2 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

此外，当我尝试安装第二个依赖项时，我说它将删除四个 remmina 包，因为我经常使用这个程序，所以我不想要这些包！