fix: reqs_optional/requirements_optional_langchain.metrics.txt to red…

…uce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-TRANSFORMERS-8400820
- https://snyk.io/vuln/SNYK-PYTHON-TRANSFORMERS-8400822
- https://snyk.io/vuln/SNYK-PYTHON-TRANSFORMERS-8400823

reqs_optional/requirements_optional_langchain.metrics.txt

@@ -6,3 +6,4 @@ nltk
 rouge_score>=0.1.2
 # below install tensorflow and downgrades numpy, so heavy dependency
 git+https://github.com/google-research/bleurt.git
+transformers>=4.48.0 # not directly required, pinned by Snyk to avoid a vulnerability