[9.20] chg: Make every changelog entry a separate code block

[9.20] chg: Make every changelog entry a separate code block

LaTeX in CI and on ReadTheDocs [fails][1] to render a PDF version of ARM if
the Changelog section is included. The running theory is that the
verbatim section of more than twenty thousand lines is too big to meet
LaTeX self-imposed constraints, and it fails with:

    ! TeX capacity exceeded, sorry [main memory size=5000000].

Or it just hangs if extra_mem_bot=30000000 is set in
/etc/texmf/texmf.d/01main_memory_bump.cnf:

    ! Dimension too large.
    \fb@put@frame ...p \ifdim \dimen@ >\ht \@tempboxa

    \fb@putboxa #1\fb@afterfra...
    l.56913 \end{sphinxVerbatim}

Make each BIND 9 release a separate code block to work around the issue.
Further split up the sections for some exceptionally large releases, for
the same reason.

[1]: https://gitlab.isc.org/isc-projects/bind9/-/jobs/4584011

Backport of MR !9266

Merge branch 'backport-mnowak/fix-arm-changelog-section-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9289