docs: cleanup pgfmanual preamble (#1300)

- Replace deprecated `ifluatex` package with `iftex`
- Drop setting input encoding to `utf8`
  `utf8` has been the new default since LaTeX2e release 2018-04-01.
- Drop setting `T1` font encoding for `dvisvgm`
  `dvisvgm` has supported OpenType fonts for years.

Signed-off-by: Yukai Chou <[email protected]>

Author: muzimuzhi

doc/generic/pgf/pgfmanual-en-main-preamble.tex

@@ -100,7 +100,7 @@
   perspective,
 }
 
-\usepackage{ifluatex}
+\usepackage{iftex}
 \newif\ifgdccodebasic
 \newif\ifgdccodeogdf
 
@@ -168,14 +168,7 @@
   % required by luatextra. Needs to be \relaxed since
   % pgfmanual-en-macros.tex defines an environment named filedescription
   \let\filedescription\relax
-  \usepackage[utf8]{luainputenc}
-  % dvisvgm does not support OpenType fonts so we have to bite the bullet and
-  % use T1 where quotes are weird.  In LuaTeX \outputmode=0 means DVI.
-  \ifnum\outputmode=0
-    \usepackage[T1]{fontenc}
-  \fi
 \else
-  \usepackage[utf8]{inputenc}
   \usepackage[T1]{fontenc}
 \fi