Unix/Linux installation: Update requirements (rebased onto develop)



This is the same as gh-712 but rebased onto develop.

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- Update Debian/Ubuntu docs to match what's provided by current releases (package names and versions) and update the Java installation notes to default to the least insecure and best supported option
- Update Java requirements
- Update Python requirements (minor rewording)
- Update Ice requirements
- Update environment variables

See the commit logs for further details of each.  The primary correction here is to remove `ICE_HOME` in the installation instructions given that it's neither used nor required.  It also corrects a number of inaccuracies and errors in the default set of environment variables documented to be required.  Most were useless, some are required under certain circumstances; this is all cleaned up and explained.

For PostgreSQL, I've updated the version details to be current, and dropped the legacy compatibility matrix, which served little real purpose for users installing/upgrading to a current version.  The matrix is retained in an updated form, but could be removed entirely if desired.

I moved a paragraph of the server installation into the client docs since it was nothing to do with installing the server.  There may be a better place for it.