doc: fix doxygen warnings

warning: Tag 'TCL_SUBST' at line 250 of file '/home/pq/build/weston-meson/doc/sphinx/doxygen.ini' has become obsolete.
         To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
warning: Tag 'COLS_IN_ALPHA_INDEX' at line 1094 of file '/home/pq/build/weston-meson/doc/sphinx/doxygen.ini' has become obsolete.
         To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
warning: Tag 'PERL_PATH' at line 2159 of file '/home/pq/build/weston-meson/doc/sphinx/doxygen.ini' has become obsolete.
         To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
warning: Tag 'MSCGEN_PATH' at line 2181 of file '/home/pq/build/weston-meson/doc/sphinx/doxygen.ini' has become obsolete.
         To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"

Observed with Doxygen 1.9.1 in Debian Bullseye.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>