Don't allow `build-base` on stable bases
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### What needs to get done
On a `platforms` charm, don't allow `build-base` to be set if the `base` is a known stable base.
This needs to include documentation about how to use build-base. See: https://github.com/canonical/charmcraft/pull/2080/files#r1919338151
### Why it needs to get done
The build base is only relevant if we can't build on the given base.
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