Require LaTeX release 2018-04-01 or later

This means we can rely on \DeclareRelease being available:
more widely the aim is to ensure users see that they will need
a somewhat up-to-date system with newer siunitx releases.

CHANGELOG.md

@@ -7,6 +7,9 @@ Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to
 
 ## [Unreleased]
 
+### Changed
+- Require LaTeX release 2018-04-01 or later
+
 ## [v3.3.22] - 2024-09-30
 
 ### Fixed

siunitx.dtx

@@ -94,10 +94,17 @@
 %
 % \subsection{Initial set up}
 %
-% Set up a couple of commands in recent-ish \LaTeXe{} releases.
 % \begin{macrocode}
-\providecommand\DeclareRelease[3]{}
-\providecommand\DeclareCurrentRelease[2]{}
+\IfFormatAtLeastTF { 2018-04-01 }
+ { }
+ {%
+ \PackageError{siunitx}{LaTeX kernel too old}
+ {%
+ You need a newer LaTeX to use this release of siunitx.\MessageBreak
+ Loading~siunitx~will~abort!%
+ }%
+ \endinput
+ }
 % \end{macrocode}
 %
 % Allow rollback to version~$2$: if we need to, version~$1$ could eventually