Drop support for older versions of D and make safe

[slook]

Mostly reverts commit 2339eea

I don't need this workaround anymore, and there's no need for us to support anything other than current runtime environments (Bookworm, etc) for this project.

$ ldmd2 --version
LDC - the LLVM D compiler (1.30.0):
  based on DMD v2.100.1 and LLVM 14.0.6
  built with LDC - the LLVM D compiler (1.30.0)
  Default target: x86_64-pc-linux-gnu
  Host CPU: sandybridge
  http://dlang.org - http://wiki.dlang.org/LDC

Thanks for doing this to help get me get up and running!

[mathiascode]

Fine with me. Have you migrated your systems to Debian Bookworm?

[slook]

Yes, I didn't need to bother doing any Bullseye deployments (other than my own system) in the end as it was easier to keep patching Stretch installations for longer than planned, so I'm targeting Bookworm from now on and will be skipping Trixie.