Modernize mod_php/Apache httpd installation guide (#5743)

* Modernize mod_php installation guide

- Add warning: mod_php should not be used for new installations;
  recommend PHP-FPM with mod_proxy_fcgi for all modern deployments
- Remove all Apache 2.2 references (EOL since 2017)
- Remove PHP 7 LoadModule example (EOL)
- Correct MPM guidance: event is the default since 2.4, prefork
  is only needed for mod_php without ZTS
- Note that major distributions already default to PHP-FPM
- Use correct project name (Apache httpd) throughout

* Remove orphaned language snippet and link to threaded FAQ.

* Attempt to make doclint happy

* Docbook nits

Co-authored-by: Louis-Arnaud <la.catoire@gmail.com>

* Docbook nits

Co-authored-by: Louis-Arnaud <la.catoire@gmail.com>

* install: fix malformed simpara/para in apache2.xml

Remove duplicated content caused by batch substitution that inserted
<simpara> blocks inside unclosed <para> tags instead of replacing them.

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Co-authored-by: Louis-Arnaud <la.catoire@gmail.com>

Author: rbowen