Sync engine/primitive requirements docs

To reduce the future hassle of syncing docs, primitive requirements in expl3.dtx
are replaced by a link to l3kernel/README.md, which is released as well.

Refer to
- df25bca (Update primitive requirements docs, 2022-02-21)
- 076d79c (Require \expanded, etc. (TL'19+), 2023-05-11)

l3kernel/CHANGELOG.md

@@ -13,6 +13,7 @@ this project uses date-based 'snapshot' version identifiers.
 ### Changed
 - Drop notes about slow `\expanded` emulation. `\expanded` is required since
   release 2023-05-15
+- Sync engine/primitive requirements in `expl3.dtx` with code and README
 
 ### Fixed
 - Case changing of Greek characters with complex NFD representation (issue \#1236)

l3kernel/expl3.dtx

@@ -964,27 +964,16 @@
 % \section{Engine/primitive requirements}
 %
 % To use \pkg{expl3} and the higher level packages provided by the
-% team, the minimal set of primitive requirements is currently
-% \begin{itemize}
-%    \item All of those from \TeX90.
-%    \item All of those from \eTeX{} \emph{excluding} |\TeXXeTstate|,
-%      |\beginL|, |\beginR|, |\endL| and |\endR| (\emph{i.e.}~excluding
-%      \TeX{}-\kern0pt-\reflectbox{\TeX{}}).
-%    \item Functionality equivalent to the \pdfTeX{} primitive
-%      |\pdfstrcmp|.
-% \end{itemize}
-% Any engine which defines |\pdfoutput| (\emph{i.e.}~allows direct production
-% of a PDF file without a DVI intermediate) must also provide |\pdfcolorstack|,
-% |\pdfliteral|, |\pdfmatrix|, |\pdfrestore| and |\pdfsave| or equivalent
-% functionality. Fully Unicode engines must provide a method for producing
-% character tokens in an expandable manner.
+% team, the minimal set of primitive requirements is currently described
+% in \href{README.md}{README.md}.
 %
 % Practically, these requirements are met by the engines
 % \begin{itemize}
-%    \item \pdfTeX{} v1.40 or later.
-%    \item \XeTeX{} v0.99992 or later.
-%    \item \LuaTeX{} v0.95 or later.
-%    \item e-(u)\pTeX{} mid-2012 or later.
+%    \item \pdfTeX{} v1.40.20 or later.
+%    \item \XeTeX{} v0.999991 or later.
+%    \item \LuaTeX{} v1.10 or later.
+%    \item e-(u)\pTeX{} v3.8.2 or later.
+%    \item Prote (2021) or later.
 % \end{itemize}
 %
 % Additional modules beyond the core of \pkg{expl3} may require additional