Update pytorch-lightning requirement

Updates the requirements on [pytorch-lightning](https://github.com/Lightning-AI/lightning) to permit the latest version.
- [Release notes](https://github.com/Lightning-AI/lightning/releases)
- [Commits](Lightning-AI/pytorch-lightning@1.8.0...2.3.1)

---
updated-dependencies:
- dependency-name: pytorch-lightning
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -5,7 +5,7 @@ boto3>=1.26.0, <1.33.8
 requests>=2.31.0, <2.31.1
 sagemaker>=2.112.0, <2.199.1
 syne-tune[aws,gpsearchers]>=0.6.0, <0.10.1
-pytorch-lightning>=1.8.0, <1.9.5
+pytorch-lightning>=2.3.1, <2.3.2
 Pillow>=9.0, <10.1.1
 tabulate>=0.9.0, <0.9.1
 tensorboardX>=2.5.0, <2.6.3