Rename security.py to security-history.py; change reference

Prep for reuse of security.txt for security reporting rather than
security design. Move old security.txt aside and change reference to it.

Author: rouilj

doc/security.txt renamed to doc/security-history.txt

@@ -3670,7 +3670,7 @@ copy.
 
 
 .. _`customisation documentation`: customizing.html
-.. _`security documentation`: security.html
+.. _`security documentation`: security-history.html
 .. _`administration guide`: admin_guide.html
 .. _`xmlrpc guide`: xmlrpc.html
 .. _FTS5 full-text search engine: https://www.sqlite.org/fts5.html