Improve error message for FIXED config options

**Describe the feature:**
When someone tries to change a config option which has the mutability level `FIXED`, then this error message is logged:

> Local setting graph.set-vertex-id=true (Type: FIXED) is overridden by globally managed value (false). Use the ManagementSystem interface instead of the local configuration to control this setting

However, `FIXED` options cannot be changed at all in an existing cluster [as the docs state](https://docs.janusgraph.org/v0.6/configs/configuration-reference/):

> FIXED: Like GLOBAL, but the value cannot be changed once the JanusGraph cluster is initialized.

It would be good to state this clearly in the error message instead of suggesting that users try the `ManagementSystem` instead which won't work.

This was just described on Discord and [StackOverflow](https://stackoverflow.com/q/73720924/6753576).

Start by searching the Java codebase for the logged message beginning "Local setting" and trace how FIXED mutability is handled. Update the message so it explains that the option cannot be changed after cluster initialization, rather than directing users to the ManagementSystem interface; done means the resulting error is accurate.

Written by the indexing model from the issue text.