Upgrade the Mono version to 5.x

Status changed to 'Confirmed' because the bug affects multiple users.

Blocked on https://github.com/mono/mono/issues/7445

This bug was fixed in the package mono - 5.16.0.220+dfsg3-1ubuntu1

---------------
mono (5.16.0.220+dfsg3-1ubuntu1) disco; urgency=medium

  * Sync with Debian (LP: #1762310, LP: #1797727). Remaining change:
    - Build with -O0 on s390x, as otherwise it fails to bootstrap.

mono (5.16.0.220+dfsg3-1) unstable; urgency=medium

  The "I'm so, so sorry, ftpmaster" release

  [ Bernhard Urban ]
  * [7ca53d0] [ppc] use ucontext_t
    `struct ucontext` has been removed from glibc and `ucontext_t` has been
    around for a while now.
    Fixes a build failure on Ubuntu 18.04
    Related glibc change
    https://sourceware.org/git/gitweb.cgi?p=glibc.git;a=commitdiff;h=251287734e89a52da3db682a8241eb6bccc050c9;hp=c86ed71d633c22d6f638576f7660c52a5f783d66
    (cherry picked from commit 9cd9e3b0a13e1f0b757606e5966e1ce27722ed95)

  [ Harlan Lieberman-Berg ]
  * [9aae867] Drop unnecessary ca-certificates-mono.postinst (Closes: #907519)

  [ aviau ]
  * [47d2e56] gbp.conf: enable pristine-tar
  * [296bc2c] create mono-source package (Closes: #785308)
  * [668e16d] d/copyright: use secure format url
  * [d339338] d/control: priority extra -> optional

  [ Jo Shields ]
  * [efc0891] Change Vcs-* entries to Salsa
  * [9960af3] Refresh rules for mcs-only DFSG'd build, post-Mono-5.0
  * [d8f9736] Refreshed symbols
  * [1be6054] Remove obsolete assemblies from installation
  * [8901434] Reference assemblies are now part of src:mono
  * [e249532] Install BoringSSL on supported architectures
  * [062438b] Don't try to calculate dependencies of reference assemblies
  * [5fd002d] Don't depend on monodoc-browser. Thanks to Matthias Fritzsche.
    (Closes: #877090)
  * [4c04777] Dropped transitional package mono-dmcs (Closes: #878770)
  * [f115a37] Delete useless symlinks in clean
  * [5021287] Delete more useless stuff on clean
  * [4be83b8] Remove package version revision from symbols (not needed)
  * [1006783] Add new assemblies to packages
  * [8c4bedd] git-buildpackage syntax update
  * [1744153] New upstream version 5.12.0.309+dfsg
  * [c5132dd] Allow binary-reference-assemblies to build with mcs instead
    of Roslyn.
    Thanks to Alex Koeplinger
  * [2b81e3e] Don't install Roslyn during `make install`
  * [d61b571] CscToolExe in xbuild should be mcs, not Roslyn
  * [d4eae1a] Try to reconcile delta between master and merge branches
  * [929eba0] Fix tab/space Makefile error
  * [d085aec] i386 and amd64 are BTLS-capable too, not just ARM & s390x
  * [dccaa7f] Fails to compile against recent glibc on s390x
    https://github.com/mono/mono/issues/9009
  * [1d77273] Remove some armel-specific symbols
  * [c8e9df1] Add a stub for mono_arch_get_interp_to_native_trampoline on
    MIPS.
    Based on 4d52a10ca048058202c39c4c2f3d05d783be9ae2
  * [8828206] A slew of s390x fixes from Neale Ferguson. Taken from
    066ce338255e0abb06b20eb3fa3526a4ccd35ab5
  * [5f87a30] btls-interface only builds on BTLS arches, not all arches.
  * [f2559a4] Fix typo
  * [830ff38] Also delete nupkgs from tarball
  * [63bd53b] Big debian/copyright update
  * [fc95e4f] Build breaks with GCC 6 and above, so use 5. (Closes: #899395)
  * [b4aecb1] Offset fix from Neale (fixes --with-tls=__thread)...

This issue is not fixed in Ubuntu Bionic Beaver 18.04 !