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

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