Upgrade of gradle-jpi plugin, jenkins version and supporting libs

[jppetrakis]

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Dependent upon: synopsys-sig/jenkins-common#34

As with pull request 33, changes made to build.gradle to address known security vulnerabilities exposed by earlier versions of jenkins-jpi, jenkins-core and other jenkins supporting plugins. The following versions are proposed:

jenkins-jpi v 0.43.0
jenkins-core 2.334 (matching in jenkins-common)

The jenkins jpi version requires an upgrade for gradle from 5.2 to 6.x (6.3 chosen).
This upgrade ALSO changes certain keywords used in the gradle file, most notably:

coreVersion -> jenkinsVersion
jenkinsPlugins -> implementation
testCompile -> testImplementaion
optionalJenkinsPlugins -> Corrected to reflect PROPER optional dependency. Behavior should be on par with original form.

[zbynek]

@jppetrakis I'm pretty sure this caused https://issues.jenkins.io/browse/JENKINS-70671, would you be able to check?

[jppetrakis]

@zbynek The gradle-jpi plugin that we needed to use has changed considerably over the last two years, and this upgrade mainly consists of using newer versions of the same, original dependencies. I do not know why certain libraries such as jenkins-core are in the hpi file now whereas not before.