docs: Make use of `text` highlighting
    
    As of 4.9.7 FreeIPA makes use of raw lexer in doc/designs/subordinate-ids.md.
    
    raw alias has been removed in Pygments 2.8.0:
    https://pygments.org/docs/changelog/#version-2-8-0
    https://github.com/pygments/pygments/pull/1643
    
    This causes the failure of Azure Docs job.
    
    I think that the original goal of `raw` was the disabling of block
    highlighting, which can be done with `text` lexer:
    https://pygments.org/docs/lexers/#pygments.lexers.special.TextLexer
    
    Fixes: https://pagure.io/freeipa/issue/8985
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>