pgjdbc: Maven -> Gradle, Java 6 => 7, JUnit 4 => 5

Hi,

It looks like its time to move forward and bump the dependencies and tools:

* Maven->Gradle

As you might know, pgjdbc is built via Maven since 2015.
It made the development, test, and release steps simpler.
It made source-based builds easier, so distributions like Fedora got
up-to-date source-based pgjdbc builds.

However, Maven has certain limitations which cause confusion and
inconvenience.
For instance, we had to split the code between pgjdbc and
pgjdbc-parent-poms repositories, which made it hard to update dependencies.

Gradle makes build scripts more powerful, and it makes it easier to
maintain the build.
For instance, with Gradle we no longer need parent-poms, the test output is
way easier to understand.

The PR is here: https://github.com/pgjdbc/pgjdbc/pull/1627

The intention is to merge the PR soon, so please let me know if you find
issues with it.

Even though the build system switch should not be a user-visible change, it
would simplify the development.
I understand it might bring disruption to the workflows, however, I hope it
won't be that bad.

* Minimal Java: 6 ==> 7 or 8

So far the minimal supported Java was Java 6. That is pgjdbc was buildable
with JDK 1.6 (end of free public updates in 2013).
I guess its time to drop Java 6, and bump it to 7 or 8.

Is there a real need to support Java 7? What do you think if we require
Java 8 and completely drop 6&7?

* JUnit: 4 => 5

JUnit5 makes it simpler to write certain test cases, however, it requires
Java 1.8 for execution.
So far I guess

* Kotlin?

* Other?

Vladimir