Centralize exit code declaration

[sormuras]

Introduce an internal collection of all status codes returned by the console launcher ExitCode. The PR also updated the User Guide to document the current state.

Addresses and closes #5051

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[marcphilipp]

Suggested change

	*/
	*
	* @since 6.0.1
	*/

[marcphilipp]

Why isn't this class package-private?

Suggested change

	@API(status = STABLE, since = "6.0.1")
	public final class ExitCode {
	final class ExitCode {