Fix XML namespace declarations

Fix XML namespace declarations

The XSL stylesheets used a mix of incorrect or outdated namespace
declarations for XHTML, probably based on ancient advice and examples.
Clean all this up.

Besides improving correctness (although probably no impact in
practice, other than possible validation failures), this removes a
bunch of useless namespace declarations in the HTML output.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/20220213201618.qz6p6noon3wagr3f%40alap3.anarazel.de