Rename the raw connection ivar to `@raw_connection`

The following CI failure seem to be resolved by merging this PR into master branch.

Run cd guides/bug_report_templates
Fetching https://github.com/rsim/oracle-enhanced.git
Fetching https://github.com/rails/rails.git
Fetching gem metadata from https://rubygems.org/.......
Resolving dependencies...
Using concurrent-ruby 1.1.9
Using minitest 5.15.0
Fetching ruby-oci8 2.2.11
Fetching ruby-plsql 0.8.0
Installing ruby-plsql 0.8.0
Installing ruby-oci8 2.2.11 with native extensions
Using bundler 2.3.8
Using i[18](https://github.com/rsim/oracle-enhanced/runs/5337457268?check_suite_focus=true#step:13:18)n 1.10.0
Using tzinfo 2.0.4
Using activesupport 7.1.0.alpha from https://github.com/rails/rails.git (at main@49299c4)
Using activemodel 7.1.0.alpha from https://github.com/rails/rails.git (at main@49299c4)
Using activerecord 7.1.0.alpha from https://github.com/rails/rails.git (at main@49299c4)
Using activerecord-oracle_enhanced-adapter 7.1.0.alpha from https://github.com/rsim/oracle-enhanced.git (at master@c[26](https://github.com/rsim/oracle-enhanced/runs/5337457268?check_suite_focus=true#step:13:26)f3ec)
/opt/hostedtoolcache/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/bundler/gems/oracle-enhanced-c26f3ec0a[29](https://github.com/rsim/oracle-enhanced/runs/5337457268?check_suite_focus=true#step:13:29)c/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:682:in `get_database_version': undefined method `database_version' for nil:NilClass (NoMethodError)

        @connection.database_version
                   ^^^^^^^^^^^^^^^^^
    from /opt/hostedtoolcache/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/bundler/gems/rails-49299c4f82db/activerecord/lib/active_record/connection_adapters/schema_cache.rb:145:in `database_version'

https://github.com/rsim/oracle-enhanced/runs/5337457268?check_suite_focus=true#step:13:1

rsim / oracle-enhanced

Rename the raw connection ivar to `@raw_connection` #2265