⬆️ bump langchain from 0.1.20 to 0.2.15

Bumps [langchain](https://github.com/langchain-ai/langchain) from 0.1.20 to 0.2.15.
- [Release notes](https://github.com/langchain-ai/langchain/releases)
- [Commits](langchain-ai/langchain@langchain==0.1.20...langchain==0.2.15)

---
updated-dependencies:
- dependency-name: langchain
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]