ImportError: cannot import name 'StaticEmbedding' from 'sentence_transformers.models'

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### Have you searched existing issues? 🔎

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### Desribe the bug

Title pretty much says it all.

```
BERTopic/bertopic/backend/_sentencetransformers.py:4
from sentence_transformers.models import StaticEmbedding
```

Looks like this is an issue with versions. SentenceTransformer didn't add Model2Vec till
https://github.com/MaartenGr/BERTopic/pull/2245
but there's no requirement that the new version of SentenceTransformer is installed.

I'll submit a PR to fix this, but I don't know what solution you'd prefer. I think the best option is to move that import from line 4 to line 58, since it appears that's the only place it's needed. I'll make a PR and test it.

### Reproduction

```python
from bertopic.backend._sentencetransformers import SentenceTransformerBackend

```

### BERTopic Version

v0.17.0

Start with bertopic/backend/_sentencetransformers.py, especially the import at line 4 and the StaticEmbedding use around line 58. Run the reported import of SentenceTransformerBackend with the affected dependency versions and inspect the project requirements. Done means the backend imports successfully without requiring an incompatible SentenceTransformer version.

Written by the indexing model from the issue text.