Maintenance release

• Add support and wheels for Python 3.15
• Drop Python 3.11 support, require Python >= 3.12
• The SLiMMutationModel is renamed SLiMv5MutationModel. The class SLiMMutationModel retains its previous behavior, but emits a FutureWarning. This change is made because mutations made by the older SLiM mutation model can be read as SLiM v6 mutations (but with meaningless values), producing a confusing situation for downstream users. New code should use either SLiMv5MutationModel or SLiMv6MutationModel, as appropriate.