Merge branch 'ma/doc-discard-docbook-xsl-1.73'

Raise the minimum required version of docbook-xsl package to 1.74, as 1.74.0 was from late 2008, which is more than 10 years old, and drop compatibility cruft from our documentation suite. * ma/doc-discard-docbook-xsl-1.73: user-manual.conf: don't specify [listingblock] INSTALL: drop support for docbook-xsl before 1.74 manpage-normal.xsl: fold in manpage-base.xsl manpage-bold-literal.xsl: stop using git.docbook.backslash Doc: drop support for docbook-xsl before 1.73.0 Doc: drop support for docbook-xsl before 1.72.0 Doc: drop support for docbook-xsl before 1.71.1