INSTALL: drop support for docbook-xsl before 1.74

Several of the previous commits have been bumping the minimum supported
version of docbook-xsl and dropping various workarounds. Most recently,
we made the minimum be 1.73.0.

In INSTALL, we claim that with 1.73, one needs a certain patch in
contrib/patches/. There is no such patch. It was added in 2ec39edad9
("INSTALL: add warning on docbook-xsl 1.72 and 1.73", 2007-08-03) and
dropped in 9721ac9010 ("contrib: remove continuous/ and patches/",
2013-06-03).

Rather than resurrecting version 1.73 and the patch and testing them,
just raise our minimum supported docbook-xsl version to 1.74.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

INSTALL

@@ -206,9 +206,7 @@ Issues of note:
    clone two separate git-htmldocs and git-manpages repositories next
    to the clone of git itself.
 
-   The minimum supported version of docbook-xsl is 1.73.
-   It has been reported that docbook-xsl version 1.73 is buggy; it needs
-   the patch in contrib/patches/docbook-xsl-manpages-charmap.patch
+   The minimum supported version of docbook-xsl is 1.74.
 
    Users attempting to build the documentation on Cygwin may need to ensure
    that the /etc/xml/catalog file looks something like this: