无法获取/安装Ubuntu 18.04的最新mono版本

我按照此官方链接安装Mono，如下所示：

成功运行了以下命令：

sudo apt install gnupg ca-certificates

然后在执行时收到：

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF

[sudo] password for rduis: 

Executing: /tmp/apt-key-gpghome.pxKRgkz7VK/gpg.1.sh --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
gpg: key A6A19B38D3D831EF: 2 signatures not checked due to missing keys
gpg: key A6A19B38D3D831EF: "Xamarin Public Jenkins (auto-signing) <releng@xamarin.com>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1

下一步：

尽管存在上述错误，mono的继续安装如下：

sudo apt install mono-devel

经过验证的mono安装版本之后：

mono --version
Mono JIT compiler version 4.6.2 (Debian 4.6.2.7+dfsg-1ubuntu1)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
    TLS:           __thread
    SIGSEGV:       altstack
    Notifications: epoll
    Architecture:  amd64
    Disabled:      none
    Misc:          softdebug 
    LLVM:          supported, not enabled.
    GC:            sgen

然而，对于Ubuntu bionic，mono的最新版本是6.12.0.122-0xamarin1+ubuntu1804b1。我只能得到旧版本的mono。我的开发需要最高版本的mono。新手换单声道。非常感谢您的帮助

sudo apt install mono-devel

mono --version
Mono JIT compiler version 4.6.2 (Debian 4.6.2.7+dfsg-1ubuntu1)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
    TLS:           __thread
    SIGSEGV:       altstack
    Notifications: epoll
    Architecture:  amd64
    Disabled:      none
    Misc:          softdebug 
    LLVM:          supported, not enabled.
    GC:            sgen