Build it with the "org.embulk.embulk-plugins" Gradle plugin

[dmikurube]

Hi @sonots,

Thanks for using/contributing to Embulk. As we discussed in the developers' meeting, we're changing some of Embulk API/SPI. At first as a preparation for those changes, let me send this PR to build this Embulk plugin with the Gradle plugin org.embulk.embulk-plugins.

One request is to set your own Maven group name here. (E.g. io.github.sonots)

group = ""  // TODO: Fill out an appropriate group.

org.embulk.* is only for github.com/embulk.

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Along with that,

• Upgrade the Gradle wrapper to use Gradle 6.2.1
• Upgrade depending embulk-core to 0.9.23 (latest)
• Set JavaCompile option for deprecation and unchecked
• Use LoggerFactory.getLogger instead of Exec.getLogger
• Remove (unused) imports to Joda-Time

It would required Embulk 0.9.14+ due to Exec.getLogger changes. But it's a year ago. I believe it would be acceptable.

[dmikurube]

Thanks!

An immediate release is not needed, but better to have soon.

This PR includes some reactions for deprecation (Exec.getLogger), and I think will have some more reactions for deprecation. Releasing a new version after them would be nice.