Remove support for java.sql types

[dernasherbrezon]

The original idea #134 was OK in 2015. Since then, Java got modular support and java.sql is no longer available by default. This is causing sporadic issues in random places:

• support java.sql.Date and Timestamp types #134 (comment)
• java.lang.ClassNotFoundException: java.sql.Time
• stackoverflow
• gson pom.xml

Would be great to deprecate support for java.sql and suggest writing type adapters instead.

[Marcono1234]

gson/gson/src/main/java/module-info.java

Line 11 in 1a9469a

requires transitive java.sql;

Would have to be changed as well, right?

[inder123]

I am not as familiar with Java 10+ (as have stayed mostly with Android). So, if someone was willing to write a PR that maintains full backward compatibilty, I will take a look.

[dernasherbrezon]

Lol, it can't be backward compatible because of "Remove support".

[Marcono1234]

I think it can be backwards compatible by using an optional module dependency (requires static) and then not trying to load any SQL classes if they are not present.
I have a local branch which should hopefully solve this, but I am not confident enough in it, so it will take a few days before I will create a pull request.

[inder123]

Wonderful!
Please write some tests as well. Backward compatibility is paramount for Gson as lots and lots of projects depend on it.

[Marcono1234]

Created #1707 now. Though I am not that familiar with the module system so any feedback is appreciated!