UpdateJavaCompatibility supports jvmToolchain shorthand

[jevanlingen]

What's changed?

• Shorthand jvmToolchain is updated.
• Support for JavaLanguageVersion.of(‹string›).

What's your motivation?

Code like

kotlinVersion = '1.9.21'
 
kotlin {
  jvmToolchain(11)
}

was not picked up to be upgraded to a newer version.

Checklist

• I've added unit tests to cover both positive and negative cases
• I've read and applied the recipe conventions and best practices
• I've used the IntelliJ IDEA auto-formatter on affected files

[timtebeek]

Nice change!