Respect config.active_record.permanent_connection_checkout

[joshuay03]

Context:

• PoC: Add an option to disable connection checkout caching rails/rails#50793
• Add config.active_record.permanent_connection_checkout setting rails/rails#51349
• https://guides.rubyonrails.org/configuring.html#config-active-record-permanent-connection-checkout
• https://github.com/rails/rails/blob/1b4484c43f32a42824d3de32e22029207f76bc42/activerecord/lib/active_record/connection_handling.rb#L273-L295
• Add ActiveRecord::Base.with_connection as a shortcut rails/rails#51083

This PR addresses two things:

1. ActiveRecord::Base.connection has been soft deprecated in favour of .lease_connection, and applications can explicitly enable the deprecation by setting config.active_record.permanent_connection_checkout to :deprecated, or disallow its usage altogether by setting it to :disallowed. In such cases the gem should not be using the deprecated method, but it would be good to avoid using it altogether so no further changes will be needed when it's hard deprecated.
2. Applications can choose to not use permanent connection checkouts altogether (i.e. by not using .lease_connection) in favour of the ActiveRecord::ConnectionAdapters::ConnectionPool#with_connection and newly added ActiveRecord::ConnectionHandling#with_connection APIs. I think it would be good to support such applications by utilising these APIs where possible, instead of leasing a connection. I've ensured that this is true for all non-test code, but I don't know of a good way to prevent further usage (see this relevant comment). Also note that ActiveRecord::ConnectionHandling#with_connection was added in 7.2, and since this gem currently supports 7.1 I've only relied on the connection pool method.

[joshuay03]

I've opted to use this constant even where we could lease a connection (i.e., here and tests) for consistency.