Ensure Linux package builds use C# 7.3 (20210817)
Ensure Linux package builds use C# 7.3 (20210817)

This will fix a recurring source of package build failures that I've
previously changed code to fix.

StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,gmartin7/myBloomFork,BloomBooks/BloomDesktop,gmartin7/myBloomFork,gmartin7/myBloomFork,gmartin7/myBloomFork,gmartin7/myBloomFork,gmartin7/myBloomFork,BloomBooks/BloomDesktop,BloomBooks/BloomDesktop,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,BloomBooks/BloomDesktop,StephenMcConnel/BloomDesktop,StephenMcConnel/BloomDesktop,StephenMcConnel/BloomDesktop,StephenMcConnel/BloomDesktop