Add OpenSSH warning to 12.4 errata page

[emaste]

This does not render as expected:

Compare with the similar note in 13.1's relnotes:

(Note the text "Important" in the 12.4 one.) @carlavilla can you advise?

[emaste]

Haven't received any feedback on tidying up the rendering so will commit as is and can be cleaned up later.

[grahamperrin]

This fixes https://bugs.freebsd.org/268199. Thanks.

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In D38999#887707, @emaste wrote:

This does not render as expected:

…

Partly related: FreeBSD bug 269819 – Tables: vertical alignment of anchored text should be flush top.

For the absence of a vertical gridline at https://www.freebsd.org/releases/12.4R/errata/#open-issues (compare with the screenshot at https://bz-attachments.freebsd.org/attachment.cgi?id=240402): we might need an additional bug report, although I think it will be more appropriate to remove the column that states 'Important'.

(I can't imagine any non-important open issue becoming an erratum alongside release notes.)

Postscript:

• ⚙ D39243 12.4R/errata: beautify the open issues area