Update mcr.microsoft images to builds on Ubuntu 20.04

[jmgrady]

Release v1.0.18 fails when exporting a project. The exception that is thrown states:

Unable to load shared library 'libdl.so' or one of its dependencies. In order to help diagnose loading problems, consider setting the LD_DEBUG environment variable: liblibdl.so: cannot open shared object file: No such file or directory:   at System.Runtime.InteropServices.NativeLibrary.LoadByName(String libraryName, QCallAssembly callingAssembly, Boolean hasDllImportSearchPathFlag, UInt32 dllImportSearchPathFlag, Boolean throwOnError)

This is identified as a problem with The Combine backend running on Docker images built off of Ubuntu 22.04 because

Ubuntu 22.04 no longer has a libdl.so library, whose functionality was absorbed into libc (https://sourceware.org/pipermail/libc-alpha/2021-August/129718.html). However, libdl.so is expected by one of our dependencies (https://github.com/sillsdev/icu-dotnet/search?q=libdl).

See issue #1768.

This PR changes the mcr.microsoft.com images used to build the backend to be images that are built on Ubuntu 20.04 instead of Ubuntu 22.04.

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This change is 

[imnasnainaec]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jasonleenaylor)

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +15.22 🎉

Comparison is base (599cb1c) 57.90% compared to head (e1bb088) 73.12%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2198       +/-   ##
===========================================
+ Coverage   57.90%   73.12%   +15.22%     
===========================================
  Files         242       42      -200     
  Lines        7747     3911     -3836     
  Branches      498        0      -498     
===========================================
- Hits         4486     2860     -1626     
+ Misses       2890     1051     -1839     
+ Partials      371        0      -371     

Flag	Coverage Δ
backend	73.12% <ø> (ø)
frontend	?

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see 200 files with indirect coverage changes

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