[bugfix] Fix getversion_nightly() function and deprecate svn support

Get nighly dump version for compat release. The version file was moved
from main to pywikibot folder in core branch. See:
https://github.com/pywikibot/Pywikibot-nightly-creator/blame/628c197c389bf2ed9431b9b9bffeacc22bffba64/nightly

Deprecate svn support because github has desupported it and no svn
checkout be done for Pywikibot any longer. Older places on
wikimedia.org are given up for years and redirects to phabricator.

• remove .svnprops property file
• deprecate version.svn_rev_info() function and remove support for svn older than 1.7
• remove version.github_svn_rev2hash() function which is no longer functional because svn support was dropped at github
• deprecate version.getversion_svn() function and remove hsh check
• fix version.getversion_nightly() to use the right path for version file
• update ROADMAP.rst

Bug: T362492
Bug: T362484
Change-Id: I8b859bab98d1ea95c4c1e9996ef827b42c1d3a61