Update project to Java 1.7 and Jakarta Mail

[bbottema]

• Java 1.6 is barely used anymore and officially not supported anymore. Time to upgrade.
• Javax Mail is also archived in favor of its successor Jakarta Mail. jakarta.mail:1.6.3 contains exactly the same as javax.mail:1.6.2 so it's a safe switch.
• Furthermore the static code analyses can be improved by applying @nullability annotations.
• Next, nowadays I automated releases to OSS Sonatype and subsequently Maven Central using CircleCI. This project can easily be upgraded for this.
• Finally, FindBugs is archived as well in favor of its active successor, SpotBugs. Time to upgrade that as well.

[bbottema]

Released as 2.1.3 (whoops). The CircleCI scripts should reset the remaining lesser versions to zero, I'll fix that. It's not a big issue right now though.

[bbottema]

Also needed for bbottema/simple-java-mail#213.