[postgres, mysql, mysql2] Update database connection semantic conventions

[dyladan]

It appears it is just drastically outdated. The spec at the time of writing the postgres instrumentation was this: https://github.com/open-telemetry/opentelemetry-specification/blob/c8109895a989480028b282fa796ed87ad1421b8a/specification/data-database.md which specified the JDBC substring.

The recent spec is quite different and requires:

• db.system - postgresql
• One of net.peer.name OR net.peer.ip - hostname or ip respectively
• net.peer.port only if using nonstandard port
• net.transport recommended in general, required only if using in-process - tcp or inproc

db.connection_string is not required but seems to be the replacement for the jdbc string.

This most likely also affects the mysql and mysql2 instrumentations

Originally posted by @dyladan in #473 (comment)