Clean up some old readme files and remove references to SVN

docs/README: Updated links, got rid of 2008 date at top of file
docs/maintenance.txt: Updated link
extensions/README: Some cleanup, added link to new git.wikimedia.org
includes/DefaultSettings.php: SVN -> Git for udpprofile
includes/profiler/ProfilerSimpleUDP.php: ditto
languages/MessagesRo.php: removed link to mime.types on SVN from the
  mimesearch-summary message. This file seems to have been deleted from the
  modern-day Git repository
maintenance/postgres/mediawiki_mysql2postgres.pl: removed SVN magic words

That takes care of most of the remaining references to SVN etc.

Bug: 38714
Change-Id: I261921df4b4c0545658d6d38c5f3c1f9dfa63ad1