Drop support for dub < v1.20 #206

Job	Run time
Default release build / Build documentation for all repos	3m 42s
Default release build / Build release for linux on ubuntu-20.04	9m 10s
Default release build / Build release for osx on macos-latest	11m 24s
Default release build / Build release for windows on windows-2019	13m 33s
Default release build / Build release for freebsd on macos-12	16m 31s
Validate outputs of build_release_template.yml	1s
Validate results of build_release_template.yml (windows-latest, windows, dmd.master.windows.7z)	44s
Validate results of build_release_template.yml (macos-latest, osx, dmd.master.osx.tar.xz)	16s
Validate results of build_release_template.yml (ubuntu-latest, linux, dmd.master.linux.tar.xz)	8s
Validate results of build_release_template.yml (macos-12, freebsd, dmd.master.freebsd-64.tar.xz)	2m 15s
	57m 44s

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