ReadMe.md: honor removal of 'frontend' branch #2755

Job	Run time
macOS--dotnet6-only	8m 51s
macOS--mono-only	7m 59s
linux22-github--dotnet-and-mono	4m 19s
macOS--dotnet6-and-mono	10m 25s
windows--dotnet6-only	6m 54s
linux22-vanilla--stockmono-only	6m 20s
linux22-github--dotnet-and-newmono	4m 33s
windows--legacyFramework-only	14m 22s
linux22-vanilla--stockdotnet6-and-newmono	6m 55s
linux22-vanilla--stockdotnet6-only	6m 0s
linux22-vanilla--stockdotnet6-and-stockmono	7m 1s
windows--dotnet6-and-legacyFramework	6m 38s
linux20-github--dotnet-and-mono	5m 41s
linux22-vanilla--newmono-only	7m 18s
linux20-github--dotnet-and-newmono	4m 40s
linux20-vanilla--newmono-only	7m 34s
linux20-vanilla--stockmono-only	6m 32s
conventions	1m 22s
snap_pkg	4m 54s
	2h 8m 18s

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