Drop support for Emacs 23

[emacs-d-mode-maintainers]

Drop support for Emacs 23

[CyberShadow]

We currently automatically test the package with 24.3, so we can safely say that anything newer than that version is supported. Although, there is currently some compatibility code with older versions, so dropping 24.3 might allow getting rid of it.

I'd say to keep the minimal officially supported version at 24.3 until we have a practical reason to drop it.

[CyberShadow]

Since the earliest version that emacs-travis.mk supports is Emacs 24.3, I suggest that that become our oldest "officially-supported" version.

Actually, since b4cbdfc, we don't even fully support 24.3 (though we do acknowledge it by disabling d-mode features that depend on Emacs features which that version doesn't provide).

[russel]

Maybe we should progress this. Is setting the floor version of Emacs at 24.4 for this mode reasonable? Or even go to 25?

[russel]

Given there have been no voices against, it seems like we have a decision. So let's declare the minimum version of Emacs to be 24.3 and tidy up the code accordingly.