Correct environment support + upgrade dependencies

[chadlwilson]

Fixes #20 by ensuring the GoCD-instructed environment is auto-registered by the agent. The plugin currently breaks the contract with GoCD which causes some confusion for users; see https://groups.google.com/g/go-cd/c/PAzY8Cw7Mxk/m/9Hbj8Fp9BAAJ

The fix is pretty simple given the current plugin design. Note that this change adds a warning in the job log if users have used the previous hacks to try and get environment support to work.

Also

• upgrades all the dependencies to ones without vulnerabilities in transitives
• otherwise ensures it can be built and work on Java 11 with modern Gradle. GocD itself has only supported Java 11+ for many years now.
• removes some dead code that has dependency on unnecessary libraries

[chadlwilson]

@adrian-iriusrisk Is this something you're willing to evaluate and merge? PR looks a little big due to rationalising/removing commons-lang and some other dependencies that came over from the plugin template this was based on, but it should be reasonably straightforward. 🙏