Update Jsch, modernize Gradle, use Groovy 3.

[elijahfhopp]

I wrote this PR while I was unaware of @gschafra 's currently open PR. This PR includes most of the improvements as #293 along with a couple more:

• Switch jsch dependency to the mwiede fork: https://github.com/mwiede/jsch (Fixes Old version of jsch is incompatible with new versions of OpenSSH servers #292)
• Update/modernize Gradle scripts to be compatible with Gradle 7.6. (Jcenter, common deps, deprecations/removals, etc.)
• Update to Groovy 3.

Backward compatibility

• Removed source compatibility block in Gradle (not necessary)
• (copypasta from test: Adapt test to updated com.github.mwiede:jsch #293) Dropped support for ssh-rsa type keys, see https://github.com/mwiede/jsch#why-do-ssh-rsa-type-keys-not-work-with-this-jsch-fork-and-my-server
• Uses Groovy 3 instead of Groovy 2.x as Gradle has moved over to Groovy 3. The code is pretty much unchanged, so it should still build with Groovy 2.x.

Next Steps

• Get CircleCI working. ✔️ (didn't stop working)
• Fix one Gradle warning: TestReport.reportOn(Object...) method has been deprecated. This is scheduled to be removed in Gradle 8.0. Please use the testResults method instead. See https://docs.gradle.org/7.6/dsl/org.gradle.api.tasks.testing.TestReport.html#org.gradle.api.tasks.testing.TestReport:testResults at 8.0.