Proposal: Retire ssl/ssl_compat.html (historical SSL compatibility page)

Hi docs@,

I've added a deprecation warning banner to ssl/ssl_compat.html in both
trunk and 2.4, and I'm planning to remove the page entirely in a future
commit unless there are objections.

The page documents directive and environment variable mappings between
mod_ssl and four products that have all been discontinued for 15-20+
years: Apache-SSL, Stronghold, Raven SSL Module, and Red Hat Secure Web
Server. The page hasn't been meaningfully updated since ~2011
(r1200006), and one of its external links (to Red Hat's Stronghold
product page) has been dead for years.

The following pages currently link to ssl_compat.html:

  - ssl/index.xml (the SSL section index)
  - mod/mod_ssl.xml (two references, both pointing readers to the
    "Compatibility" chapter for environment variable mappings)

If we remove ssl_compat, I'll update those references. The mod_ssl.xml
references just note that environment variables may be available under
different names in other products -- a footnote that's no longer
relevant since none of those products exist.

Separately, it might be worthwhile writing a new page covering practical
SSL/TLS migration topics that people actually need today -- upgrading
from older TLS configurations to TLS 1.2/1.3, modern cipher suite
recommendations, and use of mod_md for automated certificate management
via ACME/Let's Encrypt. Some of this may already be covered between
ssl_howto.html and the mod_md docs, so it might just be a matter of
better cross-referencing rather than a whole new page. Happy to hear
thoughts on that.

For now, the warning banner reads:

  "This page describes compatibility with SSL products that have been
   discontinued for many years (Apache-SSL, Stronghold, Raven SSL,
   Red Hat Secure Web Server). It is retained for historical reference
   only. If you are configuring mod_ssl on a current Apache httpd
   installation, see SSL/TLS How-To instead. This page is a candidate
   for removal in a future release."

Speak up if you think we should keep it.

-- 
Rich