Keep only the latest Rails release version information in README

Retain install and usage content for currently supported Rails versions
and drop everything older. Readers on older Rails can read the
historical README via the matching release tag on GitHub.

Also removed dead links, legacy schema examples (set_table_name,
set_primary_key etc.), the UPGRADE section (all entries were for
Rails 5.x upgrades), and unreachable Phusion Passenger blog posts
from 2008.

Clarify Oracle Database minimum tested version as 11g Release 2 (11.2);
10g is not tested in CI.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: yahonda