Question: We have upgraded our DB from 9 to 12c. I did not see ojdbc14.jar and classes12.jar. Does this mean we have to move to ojdbc5.jar, Ojdbc7.jar?

Answer: 
You should strongly consider upgrading to JDK 8. All earlier versions of Java have passed the end of their support life. JDK 9 is coming. It's time to upgrade.

The number in the Oracle JDBC JAR driver name refers to the JDK version it is written for. classes12.jar is *hopelessly* out of date - that's JDK 1.2.

There's a good reason why Oracle isn't making classes12.jar and ojdbc14.jar available anymore. 

It sounds like the best you can do is ojdbc7.jar. Get the newest JAR you possibly can and use that.

Do yourself a favor and get your Java infrastructure up to date. Using old Java with the latest Oracle version is like putting new wine in old wine skins.
I faced the same issue with my old legacy code.
I renamed the ojdbc8.jar as classes12.jar, and it is working great without any code migration.