Allow interpreters to target a specific bytecode version

[adityaanikam]

What / why

Groovy's CompilerConfiguration defaults its target bytecode to whatever JDK is currently
running it (defaultTargetBytecode() reads the running JVM's version), with no way to override
it. This means verify.groovy hook scripts get compiled to the executing JDK's bytecode level
regardless of the project's own maven.compiler.release, which breaks old source revisions as
the JDK running the build evolves. See apache/maven-invoker-plugin#742 for the full report.

This PR adds a setTargetBytecode(String) method through the interpreter stack:

• ScriptInterpreter gets a default no-op method (interpreters like BeanShell that don't compile
  to a bytecode level can ignore it).
• GroovyScriptInterpreter implements it, storing the value and applying it to the
  CompilerConfiguration used in evaluateScript(). A small normalizeTargetBytecode helper
  maps Maven-style release values ("8") to the form Groovy's setTargetBytecode expects
  ("1.8" for JDK 4-8, bare numbers from JDK 9 on).
• ScriptRunner exposes the same setter, applying it to all registered interpreters.

This only adds the capability to maven-script-interpreter. Wiring maven-invoker-plugin to
read maven.compiler.release and call this new setter is a separate follow-up PR against that
repo, since it depends on this change being available.

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